Can't delete because "some items may be locked" error?

It took a while, but I managed to get almost everything deleted by deleting things in bunches, which is how I eventually narrowed it down to that single file. I was assuming there could be hundreds, or thousands, of offending files, hidden in hundreds of folders, which would make checking every single file one at a time impossible. It would be nice if the warning dialog could be a bit more specific! When it spends a half hour "deleting" then telling you it didn't delete a thing, but you have to check all your million-plus files individually to find any permissions errors, it's certainly less than helpful! Or better yet, just delete all the files it can delete, and leave the remainder for the user to figure out.

Thanks, but I don't think that's the issue. it's been removed from Time Machine preferences for a long time. In fact it seems I can delete the individual daily backup folders. It just takes awhile. The old System folder won't delete though. That wasn't even a Time Machine backup AFAIK. I think I manually backed it up years ago before installing an OS upgrade. When I attempt to delete the folder I get the following message. I checked permissions and it appears I should be able to delete the folder itself. I used the Get Info to apply permissions to subfolders within that old System folder. But I still get this dialog. I can't check every single file within that folder individually. It would literally take years. Isn't there some way to override this? Without reformatting the drive. There's a lot of other material on that drive.

The operation can’t be completed because some items had to be skipped. For each item, choose File > Get Info, make sure “Locked” is deselected, and then check the Sharing & Permissions section. When you are sure the items are unlocked and not designated as Read Only or No Access, try again.

I may have found the main problem file. It seems I can't delete or change permissions on the file "boot.efi" which was inside CoreServices in the Library/System folder.

Yes it was, at one time. I just meant the System folder was a manual backup. But I found I could delete the Time Machine daily backups by themselves. Apparently it was the System folder that was causing the problem.