Hi there, 

To e-file, it must follow these rules. 

1. Line 3 MUST equal the total of lines 2a and 2b.
2. If the sum of lines 2a and 2b are a negative then Line 3 MUST entered as Zero. 

So IF the sum of lines 2a and 2b are a negative then Line 3 must entered as Zero and with that it breaks Rule 1 so it cannot be e-filed 

This does not mean that a taxpayer with a negative modified AGI cannot claim a Premium Tax Credit, only that the return cannot be electronically filed.

Details can be found in a link within this article: Common questions for the Form 8962 and Premium Tax Credit in Lacerte

Hope this helps!

-Betty Jo