CBT (Changed Block Tracking) not being enabled can lead to errors during VMware backups. This error message may arise if CBT is disabled after a Full Backup or the preceding incremental backup. Various scenarios can trigger this problem:

1. CBT for the VM might have been manually disabled.

2. Another application accessing the VM's CBT could intentionally disable it for certain operations.

3. An application that accessed the VM's CBT may have attempted to disable it but failed during the re-enabling process.

1. Enable CBT for the VM:

   To rectify the issue, follow these steps:

1. Power off the VM.
2. Right-click the VM, select "Edit settings," locate the "Options" tab, and click "Configuration Parameters."
3. Set the value of "ctkEnabled" to "True."
4. let the value of "scsi0:x.ctkEnabled" to "True" for each disk of the relevant VM.
5. Remove any -CTK.VMDK files from the source folder.
6. Power on the VM.
7. Power off the VM again. This step is crucial for updating the CTK table.
8. Power on the VM.
9. Re-run the backup job.
   

2. Additional Full Backup for Single VM:

If the backup job is configured for a single VM, consider running an extra full backup.

3. Additional Full Backup for Multiple VMs:

If the backup job involves multiple VMs, and all these VMs encounter the same issue, it's advisable to perform an extra full backup.

 Note: If only specific VMs among the multiple VMs in the backup job face the aforementioned issue, it's recommended not to run an additional full backup. Instead, run an incremental backup for those specific VMs.