The error "The operation can't be completed because backup items can't be modified" seems to occur when deleting files from an external drive on Mac, regardless of whether it is a Time Machine backup disk or not.
You can try the following tips to fix it.
Solution 1: Relaunch Finder
If the issue is caused by a temporary bug, restarting Finder should fix it. To do it, open Terminal and run the command below.
Solution 2: Force delete unwanted files in Terminal
You can also force delete the files in Terminal by running the following command.
rm -f /path/to/file
In Terminal, type rm -f, leave a space, then drag and drop the file you want to delete into the Terminal window and hit the Enter key.
Solution 3: Delete Time Machine backups in Time Machine
If you're trying to delete files from a Time Machine backup disk, do it from Time Machine instead of Finder.
- Keep your Time Machine backup disk connected.
- Open Time Machine.
- Locate the backup you want to remove, select it and click the gear.
- Select "Delete Backup."