The Learning Tab does not support starring yet, so you need to use the Course Overview block for that.
The course overview block will be probably disabled in your installation but it can be enabled again by an admin (and the Learning Tab can be disabled as well).
Hope it helps.
I'm the admin at the site and I've added the Course Overview block to the default Dashboard. But it shows a message that says the content is only displayed in the app.
But thanks anyway, its good to know there are no other possibilities for starring course atm, then I don't need to use more time searching for a solution.