What does this error code mean? (error code -8072).

I have just discovered that Time Machine has filled up my Raid Storage drive. Obviously I tried to delete the folders from the 'Backups.backupdb', folder selected as the TM destination. But this doesn't work and gives me no explanation of why trashing TM backup folders won't work.

This is the error that pops up after trying to move folder/files to trash.

"The operation can’t be completed because an unexpected error occurred (error code -8072)"

What does this error code mean and how do I successfully delete Time Machine's backup folders in finder?

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