i phone 8

What happens when you try? Error message(s)?

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.      

Is your phone a newly acquired phone acquired from someone other than Apple or an authorized Apple reseller? Did you are someone else erase you phone without signing out of iCloud first? In either case, it likely is in activation lock. Post back if this is your problem.