I have already seen this one. This persons solution was this:
I managed to find what I think the problem could be, in player state the unique ID was being set to nullptr due to the game mode thinking that the player was an inactive player when the PostLogin function called FindInactivePlayer when I launched the test project.
Because I was always staying within the same map while doing the testing the unique ID never got updated after successful login as this PostLogin function had already been called.
So a work around I did was to send the player to another level after successful login, now when the PostLogin function is called the logged in player is not considered an inactive one and the unique ID was set properly and I am now able to create/find/join sessions without getting the cannot map local player to unique id error.