For some reason the game won’t let you drive some of the premade tanks. Nothing a little bit of messing around in the game files cannot fix though.
Find and move the blueprint files
<- here is how you can find them if you don’t know how. Just click the gear icon in your library.
Then follow this path Sprocket\Sprocket_Data\StreamingAssets\Blueprints\Vehicles untill you reach the blueprints which will look like this:

In these files are the tanks you can face in the scenarios mode. Just go into one and pick a blueprint. For this guide i’ll yoink the Cromwell from midwar mediums


If you did everything right the tank should show up in the blueprints list now. (Yes, saving the tank will generate an icon for it.)
Enjoy your new tank!
Adding more vehicles to the Scenario Mode
Simply drag & drog the blueprint into the era you want to face it in. In this case i dropped my late war heavy into the… well… Latewar Heavies folder and now it spawns in as an enemy in the Fields map.

It does mess with the camo though but who cares ¯\_(ツ)_/¯
Thanks to Kebabiman the Kebab-Giver for his great guide, all credit to his effort. you can also read the original guide from Steam Community. enjoy the game.
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