I'm trying to file jointly, my wife's return from 2018 doesn't have the AGI number. It's completely blank. Any idea on how I could find that information?

i assume you weren't married in 2018, becuase unless you filed separate her AGI would be the same as hers.

did she file in 2018? if not hers is 0

if she did and used TT maybe your in the wrong a/c. many tt useers have multiple accounts.

as a final resource she could register at IRS and get a transcript of her 2018 return.