You can use either the Self Select PIN or last year's AGI to identify yourself to the IRS.
In the FILE section, towards the end, TurboTax asks if you want to use the same PIN as last year, but you can indicate AGI instead (or choose a new PIN).
The PIN is not a number that TurboTax has or that you can find on your return.
Here's more info on Self Select PIN.
@cvichienrat
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