You have to be 13 or older to use Pixton Solo
If you're unable to create a solo account that is likely because you're under 13 years old. In this case, you'll need to ask your teacher to create an account and invite you to join.
If your teacher is going to create an account, have them open this link: https://www.pixton.com/ and click Educator.
Teachers can create classrooms and add students to those classrooms as shown here: Creating Classrooms & Adding Students.