Time Machine could not back up the disk “Macintosh HD - Data” because a snapshot of the disk could not be created.
This is the message that I received on my iMac (late 2015) with one TB fusion drive. I also used another empty external 8TB hard drive withe the same result.
Right now, Time Machine is not creating new backups so the problem has to do with accessing the iMac hard drive??
iMac 27″, macOS 11.5