Hello,
Have you checked to see if your Default Game Map is set to the correct maps under Maps and Modes in the Project Settings? This is likely being caused by having a map set as the default that either no longer exists or due to a file path issue cannot be found.
Take a look at Project Settings->Maps and Modes and verify that the information for the Game Default Map is set to the correct map for your project.
Let me know the results.
Have a great day