Hey there,
Do you have enough space in the iCloud storage?
See this article to learn how to check it âCheck your iCloud storage online or on any device - Apple Support
If you run out of iCloud storage, your device wonât back up to iCloud, new photos and videos won't upload to iCloud Photos, and iCloud Drive, other iCloud apps, and messages won't stay up to date across your devices. And you also won't be able to send or receive emails with your iCloud email address.
See this article to learn how to manage your iCloud storage â Manage your iCloud storage - Apple Support