iPhone not backing up to iCloud
iPhone not backing up into iCloud
iPhone 7, iOS 15
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You can take a look at this for some basic help.
If you can't back up to iCloud - Apple Support
None of that may apply to you but in the absence of more details from you, I suggest that you turn off iCloud backups in the settings, close the settings app completely, force restart your phone and then try again.
This explains how to force restart. Your profile shows that your phone is running iOS 15, but you can select the correct version in the drop down menu at the top.
Force restart iPhone - Apple Support