Backup disc (LaCie) is full

My backup disc (LaCie Porsche Design P'9223 Slim Hard Drive) is full. Old backups should be deleted automatically but they don't. As a result, I cannot backup my MacBookPro (macOS High Sierra 10.13.6) any more. I have entered time machine and tried to delete some versions by simply clicking on the red circle in the upper left corner of each window, unsuccessfully. How to re-instate automatic deletion of old back up versions or force-delete old back up versions? Many thanks