Unable to install any app at all… keep getting error.

If you already tried (to no avail) ALL the steps provided in this support article:--> If you can’t download or update apps on your iPhone or iPad - Apple Support,

contact Apple Support directly, using this link:--> https://getsupport.apple.com/?caller=kbase&PGF=PGF63003&category_id=SC0078

Regards

Giulio

Turn off Screen Time.

Also check your payment card is valid, in-date, and from the same country as the phone.

Uninstall VPN if you have installed one. Preferably don't reinstall it.