Have you checked Settings/Screen Time for any restrictions?
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
If there are restrictions in Screen Time that are preventing access, you will need to restore the device to factory settings. Do not restore from a backup made when the Screen Time code was active.
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