iphone back up

Are you backing up to your computer or to iCloud? What kind of computer?

Here are two Apple support article links that may help:

How to back up your iPhone, iPad, and iPod touch - Apple Support

Keep your photos safely stored and up to date on all of your devices - Apple Support

After you have tried these if you have a problem please describe the step where the problem occurred, and any error messages displayed.

Follow this link

You probably don't have enough storage

Manage your iCloud storage

Need more iCloud storage? You can easily make space or buy more storage.

https://support.apple.com/en-us/HT204247