If the contract is still open, you can end it from the "..." menu on the contract page.
If the client already closed the contract, there will be a "Give feedback" button on the contract page, possibly on the "Terms" tab.
I checked and see your contract is still active. If the contract is completed and payment processed for the work you agreed, you can ask your client to close the contract by going to Jobs > My Jobs to locate the contract > click the (...) options menu > select End contract. You can also check this Help article for more information on how to end the contract.Once the client closes the contract, you will be able to leave feedback. Please reach back to us if you need further assistance.