There are two reasons a contest can't be closed:
1) Your contest is subject to an Admin hold. If so, you will see a message that you can contact Admin via Support.
2) You have improperly ranked according to the proper final ranking requirement. If you have one designer ranked #1, #2 and #3 etc. you will not be able to close the contest.
This is the final ranking requirement: 1) Top 3 spots (#1, #2, #3) must all be different Designers. 2) You must also have a total of 10 designs ranked.