Проблема с отрисовкой хвоста змейки PyQt5

Вы создаете, но не добавляете виджет в окно. Сразу говорю что это не решит все ваши проблемы, но виджеты добавятся (хоть и не совсем корректно). Можете дальше думать как исправлять. Удачи!

def tail(self):
    if self.snake_len != 0:
        self.element_list = [i for i in range(self.snake_len)]
        for i in range(self.snake_len):
            self.element_list[i] = QtWidgets.QWidget(self.centralwidget)
            self.element_list[i].resize(QtCore.QSize(50, 50))
            self.element_list[i].setStyleSheet("background-color: green")
            self.element_list[i].move(self.new_pos_x,self.new_pos_y)
            self.layout().addWidget(self.element_list[i])

Snake game:

import sys
import random
from PyQt5.Qt import * 


class Board(QFrame):
    msg2statusbar = pyqtSignal(str)
  
    # скорость змеи
    SPEED = 100
    # ширина и высота 
    WIDTHINBLOCKS = 50
    HEIGHTINBLOCKS = 50

    def __init__(self, parent=None):
        super(Board, self).__init__(parent)
        # creating a timer
        self.timer = QBasicTimer()
        
        # snake 
        self.snake = [[5, 10], [5, 11]]
  
        # current_x_head
        self.current_x_head = self.snake[0][0]
        # current_y_head
        self.current_y_head = self.snake[0][1]
  
        # food list
        self.food = []
  
        # growing is false
        self.grow_snake = False
  
        # board list
        self.board = []
  
        # направление
        self.direction = 1

        self.drop_food()
        self.setFocusPolicy(Qt.StrongFocus)
  
    # ширина квадрата
    def square_width(self):
        return self.contentsRect().width() / self.WIDTHINBLOCKS
  
    # высота квадрата
    def square_height(self):
        return self.contentsRect().height() / self.HEIGHTINBLOCKS
  
    # start method
    def start(self):
        # msg for status bar
        # score = current len - 2
        self.msg2statusbar.emit(str(len(self.snake) - 2))

        # starting timer
        self.timer.start(self.SPEED, self)
  
    def paintEvent(self, event):
        painter = QPainter(self)
        rect = self.contentsRect()
        boardtop = rect.bottom() - self.HEIGHTINBLOCKS * self.square_height()
  
        # drawing snake
        for pos in self.snake:
            self.draw_square(
                painter, 
                rect.left() + pos[0] * self.square_width(),
                boardtop + pos[1] * self.square_height(),
                '#5FD068'
            )
  
        # drawing food
        for pos in self.food:
            self.draw_square(
                painter, 
                rect.left() + pos[0] * self.square_width(),
                boardtop + pos[1] * self.square_height(),
                '#DA1212'
            )
  
    # drawing square
    def draw_square(self, painter, x, y, color):
        color = QColor(color)
        painter.fillRect(
            x + 1, 
            y + 1, 
            self.square_width() - 2,
            self.square_height() - 2, color
        )

    def keyPressEvent(self, event):
        key = event.key()
  
        # if left key pressed
        if key == Qt.Key_Left:
            # if direction is not right
            if self.direction != 2:
                # set direction to left
                self.direction = 1

        # if right key is pressed
        elif key == Qt.Key_Right:
            # if direction is not left
            if self.direction != 1:
                # set direction to right
                self.direction = 2
  
        # if down key is pressed
        elif key == Qt.Key_Down:
            # if direction is not up
            if self.direction != 4:
                # set direction to down
                self.direction = 3
  
        # if up key is pressed
        elif key == Qt.Key_Up:
            # if direction is not down
            if self.direction != 3:
                # set direction to up
                self.direction = 4
  
    # перемещение змейки
    def move_snake(self):
        # if direction is left change its position
        if self.direction == 1:
            self.current_x_head = self.current_x_head - 1
            self.current_y_head = self.current_y_head  
            # if it goes beyond left wall
            if self.current_x_head < 0:
                self.current_x_head = self.WIDTHINBLOCKS - 1
  
        # if direction is right change its position
        if self.direction == 2:
            self.current_x_head, self.current_y_head = self.current_x_head + 1, self.current_y_head
            if self.current_x_head == self.WIDTHINBLOCKS:
                self.current_x_head = 0
  
        # if direction is down change its position
        if self.direction == 3:
            self.current_x_head, self.current_y_head = self.current_x_head, self.current_y_head + 1
            if self.current_y_head == self.HEIGHTINBLOCKS:
                self.current_y_head = 0
  
        # if direction is up change its position
        if self.direction == 4:
            self.current_x_head, self.current_y_head = self.current_x_head, self.current_y_head - 1
            if self.current_y_head < 0:
                self.current_y_head = self.HEIGHTINBLOCKS
  
        # changing head position
        head = [self.current_x_head, self.current_y_head]
        # inset head in snake list
        self.snake.insert(0, head)
  
        # if snake grow is False
        if not self.grow_snake:
            # pop the last element
            self.snake.pop()
        else:
            self.msg2statusbar.emit(str(len(self.snake)-2))
            self.grow_snake = False
  
    # time event method
    def timerEvent(self, event):
        if event.timerId() == self.timer.timerId():
            self.move_snake()
            self.is_food_collision()
            self.is_suicide()
            self.update()
  
    # method to check if snake collides itself
    def is_suicide(self):
        for i in range(1, len(self.snake)):
            if self.snake[i] == self.snake[0]:
                self.msg2statusbar.emit(str("Game Ended"))
                self.setStyleSheet("background-color : black;")
                self.timer.stop()
                self.update()
  
    # method to check if the food cis collied
    def is_food_collision(self):
        for pos in self.food:
            if pos == self.snake[0]:
                self.food.remove(pos)
                self.drop_food()
                self.grow_snake = True
  
    def drop_food(self):
        # создание случайных координат
        x = random.randint(3, 47)
        y = random.randint(3, 47)

        for pos in self.snake:
            if pos == [x, y]:
                self.drop_food()

        self.food.append([x, y])

  
class MainWindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.statusbar = self.statusBar()
        self.statusbar.setStyleSheet("border: 2px solid black;")

        self.board = Board(self)
        self.board.msg2statusbar[str].connect(self.statusbar.showMessage)
        self.setCentralWidget(self.board)

        self.board.start()


if __name__ == '__main__':
    app = QApplication([])
    w = MainWindow()
    w.setWindowTitle('Snake game')
    w.setMinimumSize(QSize(400, 422))
    w.resize(500, 522)
    w.show()
    sys.exit(app.exec_())