Time Machine can't make any backups regardless of what disks I use

I fiddled a lot with a networked time machine disk but it took days to write just a couple of hundred gigs I think because of slow WIFI. Anyway that is not the issue I have.

I decided to buy a separate SSD to make a complete backup in case I have to restore quickly. I must have fiddled too much with the first networked disk, because now I constantly get the error:

Time Machine could not back up the disk “Macintosh HD” because a snapshot of the disk could not be created.

I get this error now on the networked disk as well, it seems to be a 'local' problem, it occurs even before anything is written to the backup disk.

This problem was described on this forum here: Time Machine snapshot could not be create… - Apple Community and the suggested solution (deleting local snapshots) seems hopeful.

However, the command for deleting local snapshots

sudo rm -r /Volumes/com.apple.TimeMachine.localsnapshots

results in the error message: Operation not permitted on my mac.

I am running Monterey 12.01 on an M1 Macbook Air. I am the only user, so I should have all the rights to do this??

I really want to have a backup because I will travel next month. Is there any solution for this issue?

thanks, Rogier