Thank you, it was really confusing:(
It is an error message on the official app.
For example, if you create and Italian armored platoon and choose the “light semovente” with the command option, if it isn’t the platoon leader the app tells you it is an invalid choice.
It just feels like an unsporting attitude, and a bit unrealistic: if they are behind a wall (or a tank in this case) how they could effectively contest or occupy an objective they cannot see?
Anyway, thanks for the confirmation about the rules.