Is it an hourly contract or a fixed price contract? If it's fixed price, you need to submit the milestone (or final milestone) first to get paid for it. If it's hourly, you can end the contract at any time. There is no 'complete contract' button, but their is always an 'end contract' option for both hourly and fixed price. A freelancer or client can always end a contract at any time. Again, if it's a fixed price contract, you'll want to submit your milestone(s) first.
Typically, freelancers wait for the client to end a contract, make any final payments, and leave feedback. Could you please try following the steps below to end your contract:
You can find more information in this help article. Let us know if you need further assistance.