SOMEONE REPLIED TO THIS ISSUE IN 2021, Solved: Anyone know how to attach IRS supplemental form 8621 to TurboTax? (intuit.com) Apparently, you need to print out the form and mail it in with your printed Turbotax return.
TurboTax does not support that form. You can complete it online and print and mail it. Here is a link to the IRS website where you can find a link to the form and instructions: Form 8621