I can not duplicate this. My dummy return has two partners. I can use 1 or 2 and I do not get the error message. Out of curiosity, what what is the first digit of the SSN? Maybe that is a common problem that needs to be reported to Lacerte.  
You might also try deleting the SSN of the Rep Partner. Exit the return. Go back in and enter the SSN again. Sometimes little gremlin ASCII codes sneak in.