I can’t backup my iPhone to iCloud
I cannot back up my iPhone to iCloud because I downgraded from iOS 26 to iOS 18
iPhone 15 Pro Max, iOS 18
You are going to have to set up your device as new. The only iCloud backup that would work to restore your data is one made with iOS 18.5. When set up as new, you are given the option to delete your current iCloud data where you will then be able to continue to use your Apple Account for iCloud backups.
Did you follow the steps here:
How to uninstall iOS or iPadOS beta software - Apple Support