You need to have an entity available in your microflow before you can use it in a REST call. Have you retrieved the token from the database, or created one in your microflow?
You must create a toke or retrieve the value from database at the MF its not a something you can get it from the session .
You need no entity but you need an object with the type of your entity. So you need to:
in your microflow before you do the REST call. Then the variable, starting with a $, is available (then you are in the right context).