https://forums.frontier.co.uk/showthread.php?t=213422
May fix itself after a while.
In the mean time I can suggest:
Try re-starting launcher.
Try validating game files for your base game.
Try restarting pc
As a last resort open the folder and delete all the test-server data and re-start the download from scratch.
The usual stuff. And see how you go. Feel free to add to the bug report linked above.
The other option is to quit everything, open up the Frontier folder (steam or otherwise) and copy the _64 folder contents into the _test server folder. This may cause more harm than good as when I did it I got an error but it may be worth a shot.