I can't add a phone number for payment

contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option.

iTunes Store - Contact Us - Official Apple Support. https://support.apple.com/en-euro/itunes-contact

iTunes Support – world wide https://support.apple.com/choose-country-region/itunes

What happens when you try? Error message(s)? More information would help.