To resolve the issue, target M365 Group need to be reviewed and decision need to be made, what should be done with it.
For example, if target M365 Group is not in use, it's much simpler to delete it, purge its Sharepoint site using PowerShell, remove the source Team and its SP from ODM UI.
Then, discover the Team again, map it (if name change is needed), then run Provisioning, then Migration. Validate that target Team got created correctly and now being shown in the Teams Admin.
However, if the target M365 Group is in use and cannot be deleted, it can be "upgraded" to the Team using this Microsoft article:
Create a team from an existing group - Microsoft Support
