After a project is marked as completed there can't be any further changes done on the project portal. You will no longer be able to add nor request additional milestones. For further services, we recommend that you offer a new listing or a new project.
The only sections that are available to use on the portal are the Chat, Review and Tips, which will be unlocked after the project is marked as completed. As a thirdSelf Member, make sure to provide all the necessary files to your client using the chat or the Files tab if it's still available to upload documents.