The error "The operation can't be completed because some items had to be skipped" indicates that the folder you're trying to delete is not allowed to be deleted by you due to permission issues.
If you have made sure your account has Read & Write access to the folder and it is not locked, try the following solutions to delete the folder:
Fix 1: Apply the permission settings to all items on the folder
- Right-click the folder and select "Get Info."
- Tap the yellow lock and enter your admin password.
- Ensure the current account has Read & Write Privileges.
- Click the Action pop-up menu and select "Apply to enclosed items."
Fix 2: Ignore ownership on this volume
Check the bottom of the "Get Info" window to see if it has an option for "Ignore ownership on this volume." If it does, tick the box next to it.
Fix 3: Delete the folder in Terminal
You can force delete the folder by running the following command in Terminal.
rm -rf /path/to/folder
To do it, first open the Terminal app on your Mac, then type rm -rf followed by a space, then drag and drop the folder to the Terminal window and hit the Enter key.
Fix 4: Format your external hard drive
If you still can't delete the folder, you can format your external hard drive to erase everything on it. Make sure you back up the important files to another storage device before doing it.