No, it does not start with a 9 and the rep is an individual.  

Sorry, no more suggestions. Just look very carefully at the input for the selected partner you have selected in screen 1 OR your input for Screen 3. You may have to contact support or use Ctrl + L and send a copy of the return and your problem.
Good Luck.

I have the exact same error.  It is the first partner on Screen 7, and I have 1 on the Client screen for tax matters. Still get the error #51439.  Also, it is not an entity, but rather an individual.  They are not electing out and I have not gone to Screen 71, and should not need screen 3, but the diagnostic jumps to Screen 3.   Strange thing is that if I put Partner 2, the diagnostic goes away-except partner two is not the TMP.  NOTE: my is in Lacerte 2018

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.

Interesting.  My partner's SSN begins with a 2.  I deleted the SSN, left the program, came back in and the diagnostic disappeared.  Entered  the SSN again, and the diagnostic reappears???? Strange.  Looks like a call to the dreaded support.
FYI:  Just did the latest update from Lacerte, and the diagnostic is gone! No phone call needed.

Glad to hear!:smile:

Just had the same problems, and of all things, I added the hyphens to the numbers and it worked.