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You cannot do 2018 or 2019 online now. Rather, you must purchase and install the 2018 and 2019 desktop software:
TurboTax® 2018 Tax Software for Filing Past Years’ Taxes, Prior Year Tax Preparation (intuit.com)
TurboTax® 2019 Tax Software for Filing Past Years’ Taxes, Prior Year Tax Preparation (intuit.com)
