Why won’t my Apple App Store let me sign in with my Apple ID?
Why won’t my Apple ID let me sign into the App Store?
Furthermore, it won’t let me download any apps.
iPad Pro, iPadOS 15
What happens when you try? Error message(s)?
Some things you can try. 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. See the section towards the end of the article.
Reset Device. -
If necessary
Hard reset. -
Do a backup and try a factory reset. Then restore from your backup.
Restore - If you see an error when you update or restore your iPhone, iPad, or iPod. *
Backup - Restore your device from an iCloud or iTunes backup. *