Students do not have the option to create their own accounts. Only Teacher and Admin account holders can create student accounts.
Student accounts can only be created by Adding the Student to a Class (if your district's SIS syncs with Otus to roster students to classes, then this option may not be available).
Did the student you would like to add have an Otus account at a previous school? If so, please contact the Otus Support Team in the chat, and we will be happy to help!