To make sure trainer works, CONFIRM THAT YOU OWN the GATHERING STORM EXPANSION. To do this in STEAM, right click the game, click PROPERTIES, then click DLC and make sure it's listed and INSTALLED.
Next in the game, click SINGLE PLAYER, then ADDITIONAL CONTENT, then MODS, then ENABLE ALL and make sure you see "Expansion: Gathering Storm" listed and ENABLED.
For EPIC, you can see OWNED next to PLATINUM EDITION UPGRADE to make sure you have the GATHERING STORM expansion, then enable it in the game as described above!
Trainer will NOT work with savegames that were saved WITHOUT EXPANSION GATHERING STORM ENABLED.
Make sure to activate the trainer in-game where you can use your units. and don't run the game as admin, only the trainer.
[Edited by DragonStryder, 9/7/2020 9:01:39 AM]
DragonStryder is indeed correct! I had the same error/issue all because I tried to activate at the main menu like many other trainers. Odds are if I looked at the readme closer it likely told me this but I didn't. Activated in-game and works perfectly!