This will occur if the individual submitted an application that was previously processed and a student record was created.

After a Student record is created from an application, that application is linked to that student record and Current Grade will be grayed out on the correponding and cannot be updated.

When a new application is submitted by the same individual you will not be able to link them to the existing application.
 
Since Current Grade must be on the Application form, trying to link it to the existing application will trigger this error when you try to process it.

Trying to create a new applicant record from the new application will also trigger an error that states that there is already an individual/student that exists with the same social security number.

An Online Re-enrollment form should be used to re-enroll students registering for new classes

If you do not have access to the Online Re-enrollment form we suggest using a Common form to re-enroll returning students. 

*NOTE: The data captured using a Common form does not transfer over the plugin.
The application data will need to be exported and manually entered into the students record in Education Edge