Time Machine backup failure

Time Machine has started to fail when I try to backup. I am running Big Sur 11.6.1 and have APFS both on my Mac and external USB drive. The Time Machine log shows:

2022-02-05 10:07:36  Not starting scheduled Time Machine backup: Automatic backups disabled
2022-02-05 10:07:36  Starting age based thinning of Time Machine local snapshots on disk '/System/Volumes/Data'
2022-02-05 10:07:36  Starting age based thinning of Time Machine local snapshots on disk '/Volumes/Macintosh HD - Datos2'
2022-02-05 10:07:36  Starting age based thinning of Time Machine local snapshots on disk '/Volumes/Macintosh HD - Data'
2022-02-05 10:07:36  Starting age based thinning of Time Machine local snapshots on disk '/System/Volumes/Update/mnt1'
2022-02-05 10:07:36  Failed to wait for snapshot deletion to complete on disk '/System/Volumes/Update/mnt1', error: Error Domain=NSPOSIXErrorDomain Code=1 "Operation not permitted"
2022-02-05 10:07:36  Skipping age based thinning of Time Machine reference snapshot 'com.apple.TimeMachine.2022-02-04-184432.local' on disk '/System/Volumes/Data'
2022-02-05 10:07:36  Skipping age based thinning of Time Machine reference snapshot 'com.apple.TimeMachine.2022-02-04-184432.local' on disk '/Volumes/Macintosh HD - Data'
2022-02-05 10:07:36  Skipping age based thinning of Time Machine reference snapshot 'com.apple.TimeMachine.2022-02-04-184432.local' on disk '/Volumes/Macintosh HD - Datos2'

Why has this started happening now? How can I solve the problem. I am not happy about not being able to make backups.

Thanks