If the issue persists, (after signing out of your Apple ID, restarting your iPhone, then signing back in) perhaps you won't be able to get rid of it, unless you:
Buy iCloud extra storage, using these instructions:--> Upgrade to iCloud+ - Apple Support
or
Manage your iCloud storage (verifying it as per this support page:--> Check your iCloud storage online or on any device – Apple Support (UK)) and decide to make room, by performing this procedure in advance:--> Archive or make copies of the information you store in iCloud - Apple Support
Regards
Giulio