Needs Battletech BETA 1.6.2 currently. You need to login to your paradox account to get the code to unlock earlier versions of Battletech on Steam.
Best to do a clean install, backup & then dump the savegame folder in \steam\userdata\steamid\637090\remote\C0\SGS1. ((STEAM))
AppData\Local\GOG.com\Galaxy\Applications\50593543263669699\Storage\Shared\Files\C0\SGS. ((GOG))
Then follow the instructions at the Roguetech wiki