In the first startup of Artifactory, in the first setup wizard you can add a license, and then when you get to the home screen of Artifactory you still get a yellow banner that says "No license found. If you have a license click here to activate. To purchase a license, visit our website or contact email@example.com." (see screenshot 1).
Then when pressing on the "click here to activate" in this yellow banner and adding the same license, you get the "License Already Exists" error (see screenshot 2).
This can be solved by performing a page refresh after adding the license in the first setup wizard (the yellow banner disappear after a page refresh).