When a tax filer passes, it's perfectly possible a federal return with their information on it may not be possible to e-file, if those handling the final arrangements have done their job. (Typically the funeral home). I can't speak for state returns however.
When someone passes, the Social Security Administration is notified of their passing (typically by the funeral home) and their social security number is locked. When the IRS computers pull data from the Social Security Administration, the IRS also gets that notification of the deceased. The IRS then also locks that SSN so that it can't be used fraudulently via electronic means with the IRS. When attempting to e-file a return where one of the tax filers is deceased the most common error is "SSN LOCKED". So if you get that error, the only option you may have is to print, sign and mail the return to the IRS.
