iphone

Are you trying to back them up or sync them to iCloud?

Try Settings/Your Name/iCloud and slide off syncing. Wait a few minutes and then slide syncing on. Allow time for syncing to occur.

Last resort - have you tried signing out of iCloud and then sign back in?

Contacts - Troubleshooting iCloud Contacts

Try https://support.apple.com/guide/icloud/restore-or-set-up-your-ios-or-ipados-device-mm7e756df7fd/icloud