I know this is an old thread but I have the same issue, with a new 12 pro.
Have not been able to backup since restoring the last backup on it.
Given that iTunes is not being used anymore, only the finder and I see this:
Could not back up the iPhone because not enough free space is available on this Mac.
Removing files and emptying the Trash will free up additional space.
I deleted the oldest backups and cleared the trashcan and I still have ~430gb free. Cannot tell why I see this error.
I tried First Aid via Disk Utility and it did nothing.
edit: I moved the backup files to a temp folder and made a fresh backup and it worked.
Possibly an issue using an old backup file that was made with the previous phone.