The message "A horse is not registered, the related result will be flagged as "not registered" and won't count in any ranking" appears if the concerning horse is not correctly registrated by the National Federation.

Probably causes:
A. The horse has been entered manualy and the FEI ID is not correct. 
                         ==> Enter the correct horse FEI
B. The registration for the current year has not been done. (Check the horse in the FEI database)

==> The rider of PR needs to contact the NF to have this issue solved.