Yes, I'm having trouble with that error too.  The form 3468 instructions say "If applicable, enter your pre-filing registration number..." so I'm thinking it is not required, but I still get the error if I input "Not Applicable". The taxpayer says they don't have an IRS registration number. 

Also, when I enter the cost of the solar energy property on form 3468, Part VI, Line 3a, the cost does not flow to the K-1s on the 1065 and 1120S modules in Proseries.