This should help: https://www.irs.gov/individuals/electronic-filing-pin-request
The IRS uses it (or last year's PIN) to validate your return. (Note, if your spouse died last year, you cannot use AGI for validation. This is due to a problem with TT and there are numerous threads from over the last few year's about this.)
In order to e-file you will need your 2019 AGI. Please click here to learn how to find your 2019 AGI.
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