Canvas uses Google to log in (known as Single-Sign-On or SSO).
Information for students is pulled nightly from Synergy.
If a student cannot log in to Canvas, it is almost universally for one of the following two reasons:
1) The student's Google account has not been created yet. This can take 24-48 hours from the date the student is enrolled within Synergy.
2) The student's email account creation has failed, because their preferred name is entered incorrectly or contains diacritics or other invalid characters. (Please see How are student GMail accounts & passwords created?)
2) The student's Email address is not yet entered in Synergy, or is entered incorrectly. Email addresses are generated based on preferred name, and are synced back to Synergy overnight.
To expedite things, please have your school Registrar verify that the student's email is entered correctly in Synergy.
If you need to know what a student's email address will be please read: