You're going to have to print and mail the return anyway so just ignore the message and proceed.
HOWEVER, after you print out the return, make sure you write "deceased", the person's name and the date of death across the top of the 1040.
DECEASED: [Name of Decedent] - [Date of Death]
Sign the return as the personal representative and file Form 56 for good measure. Include your court appointment as personal rep (or letters) if applicable.
