When a district is Managed by SIS educators are restricted in certain functionality, including adding a new student. The only option to manually add a student is to create an "un-managed" classroom (see Create a Classroom) and add the student (see Add a New Student).
If this student will be managed once entered into the district's SIS, we advise that when adding the student, ensure the Student Information Number is aligned to what is your district's SIS. Then, once the student is added to the SIS, they will be successfully moved (along with their work) to the correct classroom.
Please note: This is only recommended when a student will be added to the SIS within a short period of time. With SIS integration, there is a risk of students being deactivated and student work being lost if using this functionality in a non-standard way.