"Time Machine could not back up the disk “Macintosh HD” because a snapshot of the disk could not be created."
that does not sound good, at all
...
975GB/1TB used
Discussion
There was a “Verify Disk” option in disk utility app… Maybe check option & right click on backup symbol in task bar to verify your existing backups… https://osxdaily.com/2012/02/08/repair-boot-disk-mac-os-x-disk-utilit/
Time machine was such a shitshow in my experience. Disconnection, need restart to work again, stop backup for no reason without much warning.