If your student is unable to log in, the first thing to check is whether they are properly activated in your class.
From the Manage classes menu on your homepage, select the class in which the student should be activated.

If they do not appear in your student list, it means their account is not activated.
Please refer to the following article to add them to your class: How do I add a new student to my class?
If the student appears in your class list but is still unable to access their account, it’s important to check the following information:
Is your student logging in using their full name and a password?
If so, make sure their name is correctly entered in their profile and that they are typing it correctly when logging in.
We also recommend changing their password by following this procedure: How do I change the password of an activated student in my class?
Make sure the new password is different from the one used previously.
Is your student logging in using their email and a password?
If so, ensure the email address entered in their profile is accurate and that they are typing it correctly when logging in.
We also recommend changing their password by following this procedure: How do I change the password of an activated student in my class?
Make sure the new password is different from the one used previously.
Is your student using a Google or Microsoft login?
If so, make sure an email address is entered in their profile.
If not, you can add one by clicking on the three dots to the right of their name in the student list.