Как добавить тег в XML?

Добавил пример парсинга XML, нахождения определенного тега mPINTABREQUESTDetailType и добавления в него нового тега CTX_files

Пример:

import xml.etree.ElementTree as ET

text = """\
<?xml version='1.0' encoding='UTF-8'?><S:Envelope xmlns:S="http://schemas.xmlsoap.org/soap/envelope/"><S:Body><ns2:PinTabrequestResponse xmlns:ns2="http://temenos.com/CardPinTabRequest"><Status><successIndicator>Success</successIndicator></Status><PINTABREQUESTType><gPINTABREQUESTDetailType><mPINTABREQUESTDetailType><CardNumber>6612644511582146</CardNumber><ContractType>Yes</ContractType><CardNumber2></CardNumber2><NameSurname>VOLKOVA</NameSurname><PersonalCode>1</PersonalCode><CustomerId>9</CustomerId><UserNameSurname></UserNameSurname><UserPersonalCode></UserPersonalCode><UserCustomerId></UserCustomerId><Language>2</Language><Residence>LV</Residence><Street>BULVARIS 60-15</Street><KeyPhrase>E</KeyPhrase><Sector>6000</Sector><AccountOfficer>762</AccountOfficer><Sex>2</Sex><RemoteServices></RemoteServices><PasportNumber>329</PasportNumber><PassportExpire>20</PassportExpire><PhoneNumber></PhoneNumber><MobileNumber>5</MobileNumber></mPINTABREQUESTDetailType></gPINTABREQUESTDetailType></PINTABREQUESTType></ns2:PinTabrequestResponse></S:Body></S:Envelope>
"""

root = ET.fromstring(text)

detail_type_el = root.find('.//mPINTABREQUESTDetailType')

ctx_files = r"C:\foo\abc\123.xml"
ctx_files_el = ET.Element('CTX_files')
ctx_files_el.text = ctx_files
detail_type_el.append(ctx_files_el)

xml_bytes = ET.tostring(root, encoding="utf-8", xml_declaration=True)

with open("fileFromT24.xml", 'wb') as f:
    f.write(xml_bytes)

Если нужно именно добавлять в файл, как ранее до этого делали, тогда вместо wb используйте ab+. Можно оставить и a+, но нужно будет из байтов в xml_bytes получить строку xml_bytes.decode('utf-8')

Добавил пример кода на коде из вопроса, постарался сохранить максимум логики старого кода:

import xml.etree.ElementTree as ET


def send_to_t24(card, ctx_files: str):
    url = "http://xxxx.xx:8900"
    header = {"content-type": "text/xml"}
    body = """
    <soapenv:Envelope xmlns:soapenv="http://xxxx.xx:8900"
    <soapenv:Header/>
    <soapenv:Body>
    <car:PinTabrequest>
    <WebRequestCommon>
    <company>1</company>
    <password>Q1</password>
    <userName>O</userName>
    </WebRequestCommon>
    <PINTABREQUESTType>
    <enquiryInputCollection>
    <columnName>PAN</columnName>
    <criteriaValue>CARD</criteriaValue>
    <operand>EQ</operand>
    </enquiryInputCollection>
    <enquiryInputCollection>
    </enquiryInputCollection>
    </PINTABREQUESTType>
    </car:PinTabrequest>
    </soapenv:Body>
    </soapenv:Envelope>
    """

    post_data = body.replace("CARD", str(card))
    sendrequest = requests.post(url, data=post_data, headers=header)

    text_xml = sendrequest.text
    root = ET.fromstring(text_xml)
    detail_type_el = root.find('.//mPINTABREQUESTDetailType')
    ctx_files_el = ET.Element('CTX_files')
    ctx_files_el.text = ctx_files
    detail_type_el.append(ctx_files_el)

    xml_bytes = ET.tostring(root, encoding="utf-8", xml_declaration=True)
    text_xml = xml_bytes.decode('utf-8')

    convert = open("fileFromT24.xml", "a+")
    print(text_xml, file=convert)
    convert.close()

...


for value in read_file_ctx():
    ctx_files = value
    # csv_files = str(value).replace(".ctx", ".csv")
    with open(ctx_files, errors='ignore') as crd:
        for line in crd:
            if "%B" in line:
                card = line[9:25]
                # print(line[9:25])
                send_to_t24(card, ctx_files)