Before you can post your lesson to Google Classroom, you need to make sure at least one card in your lesson is visible on your class website. You don't need to have a class website or even be a Cc Pro subscriber to use the Share to Google Classroom button. You just need at least one card in your lesson plan to say "Visible on Class Website".
Cc makes you do this because it will only share cards that say "Visible on Class Website" to Google Classroom. That way you get to choose what shows up on Google Classroom and what doesn't. So you could, for example, make "Homework" visible on Google Classroom, but not "Accommodations and Modifications".
Click here to learn how to show a card on your class website.