... make sure the characters are correct: 0 vs O, I vs 1 vs l, etc.
... ensure there are no preceding or trailing spaces.
... if it still won't work, consult your instructor or your class materials.
If your teacher didn't give you a Course Key...
... consult your teacher or class materials for your Course Key.
OR
... confirm whether you're supposed to access OLI with a single sign-on link on your school's website (a learning management system such as Blackboard, Canvas, D2L, etc.).