U090 – You do not have access to the assignment in Google Classroom where this link was posted

BookWidgets assignments used in Google Classroom are intrinsically connected to both the teacher that uploaded them and the course they were uploaded in.

If you receive the error message above as a student, then it means there is a mismatch between your Google login and the course in which the BookWidgets exercise was shared. More specifically, it can mean the following:

You are signed in with a student account that does not have access to the course in which the BookWidgets assignment was originally shared and need to sign in with a different account in order to gain access.
Your teacher originally shared a BookWidgets assignment in a Google Classroom course in which you're not enrolled. After sharing, the assignment was copied from the original Google Classroom course into the one from which you're trying to open it.

If you are sure that you're signed in with the correct student account, then please request your teacher re-share the widget in the correct course. As the message suggests, you can open the widget exercise regardless, but your grades will not be able to sync with your Google Classroom grade book.

For teachers, continuing regardless of this warning additionally entails the following:

Student activity cannot be monitored in the Live-overview;
Submissions will end up in the wrong course;
A warning/error message will appear next to the students' submission in the Review section of your account, indicating that the grades can't be synchronized.