Based on their birthdate, the student was over age 21 on the first day of the school year.
This error is a ‘head’s up’ to the district indicating they have a student who was 21 on the first day of the school year. This might be accurate.
If the student really is 21, no problem. Comment the error (as acknowledgement) and leave it be.
Now, if the student really isn’t 21, there’s likely a problem with their date of birth in ADVISER Validation. Since this information is actually coming from the ADVISER Person ID collection (versus your SIS), you’ll need to go to the ADVISER Person ID collection within the NDE Portal and adjust the information within it. You may also need to update your SIS, but the information in ADVISER Validation is not dependent upon that update.