Only 2020 can be prepared online and efiled. Earlier returns require that you purchase and install the TT desktop software for the applicable year, and file by mail. See this for 2019, for example:
TurboTax® 2019 Tax Software for Filing Past Years’ Taxes, Prior Year Tax Preparation (intuit.com)
Also note that the Statute of Limitations to file for a 2017 refund expires today (May 17, 2021).
