When this issue was encountered, the event registration was for a group and the primary registrant (also known as the host/donor) had multiple possible matches, with no constituent linked in the Linked constituent field or Linked constituent auto-match field. 

To fix this issue, link the host/donor to a constituent record. If the incoming registrant's information matches that of an existing constituent, select the existing constituent in the batch's Linked constituent field. If the incoming registrant's information does not match that of the existing constituents, create a new constituent record with the incoming registrant's information, then select the new constituent in the Linked constituent field.