
you can manually edit the path, save and launch again
if, for whatever reason, one is not blank but contains an invalid path, then you get that error message.
if it is blank ("") for all games defined in the list, the program should try to detect the games location and fill the list
you are only interested in the ExePath parameter. in GameData.xml look at the lines starting with 712100'. #MOD MANAGER DOWNLOAD ROME TOTAL WAR 2 ISSUE MOD#
Also, and this is very important, you need to launch the game at least once after installing it through Steam, let the additional setup take place, let the launcher start, (add a mod or not), launch the game, quit from the menu and only then start the mod manager and try to detect it.
Reasons for not detecting your game could be non standard location (like custom steam library) or changing your drive letters or moving a game after it was previously located. 2nd xml should be generated after you successfully launch the mod manager and save a profile or launch a game. If not, ask again, provide more context and maybe we'll figure it out together. I am just a normal user, not associated with the development team, so what follows is just what I figured out for myself.