If the TM Backup Drive is formatted in APFS / GUID Partition Map format - not possible to cherry pick individual Snap Shots to delete.
The Drive is also Marked by the Operations System as Read Only
The only true options>>
1 - Set aside the existing Drive after removing the Drive from the TM Backup Utility
2 - Then get a Larger Drip with about 2.5 x Greater Drive Capacity than the Internal Drive and set this new drive up as New
3 - Ding the above will still allow you to access the Old TM backup Drive to restore files
4 - Reformat the Existing drive and start again
A Good Suggestion from another user and look for the response from @steve626 on removing the drive from TM Utilities and /or reformatting the existing drive.
Time Machine says that my GDrive mobile U… - Apple Community