global не работает. Не пойму в чем проблема?

Попробуйте так:

from tkinter import *
import random

window = Tk()
window.title('Game')
window.geometry('900x800')                       
window.resizable(width=False, height=False)

canvas = Canvas(window, width=900, height=700, bg='blue')     
canvas.pack()

side = Label(bg='red')                         # black
side.place(x=0, y=600, width=900, height=10)   # x=0, y=700, width=900, height=10


class Orb:
    def __init__(self):
        self.x = random.randint(25, 875)
        self.y = random.randint(25, 675)
        self.v = -1
        self.v1 = -1
        self.photo = PhotoImage(file="_+_.png")  # установите свое
        

class Block:
    def __init__(self, hp):
        self.x = random.randint(50, 850)
        self.y = random.randint(50, 650)
        self.hp = hp
        self.photo = PhotoImage(file="Ok.png")   # установите свое


blocks = [Block(1),Block(1),Block(1)]
orbs = [Orb(),]                                  # [Orb(),Orb()]  ???

counter = 0
speed = 10


def game():
    global speed, counter
    window.after(speed, game)
    canvas.delete('all')
    for orb in orbs:
        canvas.create_image(orb.x, orb.y, image=orb.photo)
        orb.x += orb.v
        orb.y += orb.v1
        if orb.x < 25:
            orb.v = orb.v * (-1)
            orb.x = 25
        if orb.x > 875:
            orb.v = orb.v * (-1)
            orb.x = 875
        if orb.y < 25:
            orb.v1 = orb.v1 * (-1)
            orb.y = 25
        if orb.y > 675:
            orb.v1 = orb.v1 * (-1)
            orb.y = 675
        for block in blocks:
            canvas.create_image(block.x, block.y, image=block.photo)
            if ((orb.x - block.x) ** 2 + (orb.y - block.y) ** 2) ** 0.5 < 75:
                print(f'counter = {counter}') # 
                counter += 1
                blocks.remove(block)
                blocks.append(Block(1))
                orb.v = orb.v * (-1)
                orb.v1 = orb.v1 * (-1)

# +++
                money.config(text=f"$: {counter}", bg='#00FFFF')      # +++

def ok():
    hello.destroy()
    hello_button.destroy()
    game()

speed_up_money = 10
count_speed_up = 1

def speed_up_func():
    global counter, speed_up_money, speed, count_speed_up
    if counter >= speed_up_money:
        count_speed_up += 1
        counter -= speed_up_money
    if count_speed_up == 1:
        speed = 10
        speed_up_money = 10
    if count_speed_up == 2:
        speed = 7
        speed_up_money = 10
    if count_speed_up == 3:
        speed = 5
        speed_up_money = 30
    if count_speed_up == 4:
        speed = 3
        speed_up_money = 90
    if count_speed_up == 5:
        speed = 1
        speed_up_money = ''
        speed_up_button.destroy()


hello = Label(text="Цель игры - купить все улучшения\n(серые круги = $)",
    bg="grey77", font="Arial 24", relief="raised")
hello.place(y=300,x=200)
hello_button = Button(text="Ok", command=ok,font="Arial 14")
hello_button.place(y=380, x=675)

speed_up = Label(text=f"Улучшений скорости: {count_speed_up}/5\nУлучшить скорость: {speed_up_money}",font="Arial 14")
speed_up_button = Button(text="Купить",command=speed_up_func,font="Arial 12")
speed_up.place(x=200, y=610)
speed_up_button.place(x=200, y=660)

money = Label(text=f"$: {counter}")
money.place(x=50, y=650)


if __name__ == '__main__':                                   # +++
    window.mainloop()                                        # +++

_+_.png

Ok.png