I have a fixed contract. I have sent work for the first milestone. Should I close the milestone once the milestone is cleared? or There will be any negative effect of doing so?
You are free to end a contract at any time. Typically, freelancers wait for the client to end a contract, make any final payments, and leave feedback. If you end the contract and the client doesn't leave feedback, it will not be counted in your Job Success Score calculations. So there will be no negative or positive effect.