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