I'm buying a house and the mortgage company is asking for a "signed copy" of my 2018 tax returns. They won't accept that I filed via TurboTax. Can anyone help?

Print a copy of the 2018 tax return, Form 1040, sign in ink and give them the copy.

signed by whom?  you could always print out a copy and sign it.   Tell them it was self-prepared.  otherwise tell the mortgage company they're more stupid than earthworms. find a new one.    There are many that don't have a clue that a self-prepared efiled return won't have any signatures.