Thank you for your message. To release the funds on the milestone, you need to click 'Review and Pay' on the contract page and then click 'Approve & Pay' once you have reviewed the Freelancer's work submission. You can refer to this article for step-by-step instructions on making a milestone payment.
I checked again and it looks like your 3rd milestone is not approved yet. You will need to approve this active milestone by clicking 'Review and Pay' to release funds before you create and fund the next milestone on the contract.