Can't delte the files on my WD External Hard Drive

I have a 500GB WD external harddrive and I have used it to back up my iMac using Time Machien. I have stoped using time Machine, because the hard drive got full and so now I wanted to clear it out but it will not let me. Any ideas on how to fix this?

What happens ?  What message do you get?

Was the backup encrypted?