Tax Return for Deceased Mother

I just received a Form 3531 from the IRS for my mother's tax return (she passed June 2024) which I submitted using TurboTax.  The Form 3531 states:  "we require a power of attorney or court certificate in all instances when someone other than the taxpayer is signing the return."  I am unsure what I need to do.  Ant advice would be most appreciated,