You may need to explain more.    Are you saying your husband did not file a return at all in 2017?   Were you not married yet in 2017?  

Did you marry in 2018?  Or in 2017 were you married and filed a joint return?

We were not married. His parents claimed him and he had no income for the year.

If your husband did not file a return at all in 2017 when you were unmarried, then he has no 2017 AGI in the IRS database, so he would use an AGI of 0 (zero.)

I put zero and it rejected the return.

What is the exact rejection error code or message?  

If it was HIS AGI that rejected, and he filed no return in 2017,  then there is no other choice unless his parents filed a 2017 return in his name, in which case there would be a 2017 return transcript at the IRS.  But you said above that he had no 2017 income, so I can't see that they would do that.