from a earlier post:
Yes. You can generally e-file the currently due tax year and two prior years. Currently, tax year 2019 and tax year 2017 returns can be e-filed. Tax year 2018 federal e-filing will be available in late February, due to IRS extender updates passed recently. Extension filings are only available for the currently due tax year.
The last day to Efile in 2020? Usually mid November sometime the IRS shuts the Efiing season down for individual returns until they reopen in Jan 2021...I think business returns get until December sometime (I dont file any of those so Im not as familiar with that cut off date)