There are many IRS Forms not supported by TurboTax - https://ttlc.intuit.com/community/forms/help/which-irs-forms-are-not-included-in-turbotax/00/25551
There are also certain tax situations and calculations not covered by TurboTax - https://digitalasset.intuit.com/DOCUMENT/A7aTcVUwu/2018-unsupported-calcs-L1FQfTTgn-073019.pdf
IRS Form 926, Return by a U.S. Transferor of Property to a Foreign Corporation would not be an IRS Form that would be supported since it is applicable to very few taxpayers - https://www.irs.gov/pub/irs-pdf/f926.pdf
