As you stated, TurboTax does not support IRS Form 8833. If you need to include this form with your tax return and the return has already been filed and accepted by the IRS then you will have to file an amended tax return, Form 1040X. Before amending the tax return wait for the IRS to send you the tax refund or if you have taxes owed then for the IRS to process the tax payment.
See this TurboTax support FAQ for amending a 2016 tax return - https://ttlc.intuit.com/questions/3061514-how-do-i-amend-a-2016-return-in-turbotax
IRS Form 8833 Treaty-Based Return Position Disclosure
Under Section 6114 or 7701(b) - https://www.irs.gov/pub/irs-pdf/f8833.pdf
Also see this IRS website for claiming Tax Treaty Benefits - https://www.irs.gov/individuals/international-taxpayers/claiming-tax-treaty-benefits
