Looks like the teacher that shared a link with you is in a different region. To access this content you'll need to switch your region.
To switch regions, please refer to the following steps:
Log out of the app or website
Click on the button in the top right within account selection (at sign up)
Now you can select from the other regions
You should now be able to open links from that teacher.
Note: Depending on your teacher's region, you may be required to sign up in their region to access their classes.