Can you add to your question a screenshot of the page in which you have to enter an API-endpoint, please. Now it is unclear (at least to me) if you are either a) switching from a free node to a paid license on the Cloud or b) uploading a non-version-controled app to a version controlled app or c) doing something else.
Thinking again, i think i know where you are stuck: deploying your locally running app to the Mendix cloud on a Free node. And I think you have not yet created your first branch yet. So do this in MxStudio Pro: click Menu ‘Version control’ → ‘Manage Branchlines’ and then in box ‘Branch lines’ click button ‘New’. Give it name ‘Main’, save it and off you go.
Best of luck!