If you are trying to share an assignment in Google Classroom and it doesn't seem to be working, this might be due to filters from your browser. Look to see if there is a red 'X' on the right side of your browser search bar, click on it and select "Always allow pop-ups".
Here are articles to explain how to enable pop-ups on:
Google Chrome: https://support.google.com/chrome/answer/95472?hl=en
If pop-ups aren't being blocked and you are still having this issue, please contact us at firstname.lastname@example.org and we'll look into the issue for you!