If you want to use a deck of Boom Cards as an assignment in Google Classroom, you'll need to set it up properly first.
If you do not need your student's performance tracked, and only want to use the deck as practice or review, simply create a Fast Pin in Boom Learning, create a new assignment in Google Classrooms, then click Add, then Link, and copy-paste the Fast Pin link into the text box.
If you want detailed student reports, you'll need to assign the deck to the class first. After Importing a Google Classroom, select the Google Classroom from the left side of the Classes page. Then select the "Assignments" sub tab and click the "assign more decks" button to see a full, searchable list of all your decks.
You can assign/un-assign by clicking the checkbox next to each deck displayed.
Once your deck is assigned to the class, go to your Library tab and click the Action dropdown menu for the deck you want to assign, then click "Hyperplay Link." If you give students a Hyperplay link without Assigning the deck first, the students will not be able to see their Student Report for the deck.
Once you have the Hyperplay link, create a new assignment in Google Classrooms, then click Add, then Link, and copy-paste the Hyperplay link into the text box.