You get an error message that the student could not be registered because they need an assessment before enrollment.
Some states have customizations in place that require students to have an assessment prior to enrollment in a class/group.
Check the documentation regarding state-specific customizations, available in the User News link of your Person Profile (the head with the circle around it in the upper right-hand corner)
If your agency is in a state requiring assessment prior to enrollment, insure that your student has a valid assessment resulting in a valid entry Educational Functioning Level within the fiscal year or 90 days prior to enrollment.