Why do I see "No Access" on an exercise?

This usually means you are signed in with a different email from the one your school added to the class. Click your profile picture, then Me, to check which email you are using.

If it is the wrong one, sign out of your other accounts or open a private (Incognito) window, and sign in again with the right email. If the email is right, ask your teacher to check that you are on the class list.