I can't add my credit card it says An unknown error has occurred

Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch

Try a forced restart.

Reset Device

If necessary

Hard reset

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting

Backup - Restore your device from an iCloud or iTunes backup

If necessary, 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 Support

iTunes Support – world wide

Hello.

Have you tried to add it in settings? If no, try it in settings > name surname at the top > iTunes&App Store > click blue email address > view Apple ID > payment methods.