You can use the 2019 Form 1065 if:
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The partnership has a tax year of less than 12 months that begins and ends in 2020, and
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The 2020 Form 1065 isn't available by the time the partnership is required to file its return.
You can use the 2019 version of TurboTax Business, but you (a) will not be able to transfer your (already filed) 2019 return into the program and, (b) you will have to override the dates on certain forms (such as the 1065 and K-1s), which will generally involve printing and mailing the return.
Also note that you can use the 2019 tax return that you prepared (and filed) with TurboTax Business (i.e., use the .tax2019 file) to get the basic information into the program - just make sure you save a few copies of that tax file on other media and do not overwrite it.
