This error will occur because you have stated that you are using Stockbook as a Breed Society in the Setup user options.
If you are not using Stockbook to run a breed society, this needs to be changed to a Normal User (even if you run a stud operation).
If you are using Stockbook to run a breed society, the error indicates that you do not have a member whose herd letters match the first part of an animal identification that you have entered.
1. Change User Options