I can’t backup my phone

This assumes your iPhone is running 13.7 (current) iOS.

If not, update to that, and try again.

Then back up locally, using a Mac or Windows system.

This so that you have a backup.

Always have a backup.

Then log into iCloud.com directly, or use the following sequence, and try to delete the problem backup.

Remove a device backup from iCloud - Apple Support

You are given the reason in your screenshot. Perhaps you can explain it to us.

”The last backup could not be completed because iCloud contains a backup from a newer iOS version.”