This message appears when the rider is not entered (or not entered in the correct category) in the FEI entry system of the current show.

Check the FEI entry system if the rider has not been accepted.
If the rider is not correctly entered inform the concerning rider that he needs to inform his NF immediately.
You can also use the FEI entry system to send an email to the concerning NF to ask them to change/add the entry.

Search the concerning rider in the FEI entry system
In the Athlete section select 'Contact entering NF...'

Select the reason for contact:

"Event change request"  Entry has been changed from event (eg. from CSI1* to CSI2*)
"General" for all other requests (eg. horse and categorie change)