I can't delete the trash, I get error message that "items are in use"

You should never ever move Time Machine backups to the Trash.

The way to fix the problem, is to erase your Time Machine backup drive and start over with new backups.

You could also eject, disconnect and set that external drive aside, then start Time Machine backups on another external drive.

Or you can just start another drive as an ADDITIONAL backup set, without removing the old one. Time machine will create a new stand-alone backup on the new drive starting from today, and every-other backup will go to every-other drive until you re-configure.

it's not a lot more money to buy a drive that is a bit larger.