I'm using the same setup with Jaspera Sentinal and Magda, Brazen Outlaw - it seems that Jaspera Sentinal only gives you the option to tap when the game thinks that there's something you could pay for with the tapped mana. For example, if I have a 1 cost instant in my hand and an available creature to tap, I can always tap Jaspera. But if I have nothing in my hand and want to tap at will for treasure token generation it doesn't work.
I'm using the same setup with Jaspera Sentinal and Magda, Brazen Outlaw - it seems that Jaspera Sentinal only gives you the option to tap when the game thinks that there's something you could pay for with the tapped mana. For example, if I have a 1 cost instant in my hand and an available creature to tap, I can always tap Jaspera. But if I have nothing in my hand and want to tap at will for treasure token generation it doesn't work.