The IRS requires your 2019 AGI to verify your identity for e-filing. If you're paper-filing your return, you won't need your AGI because those are manually processed.
There are a few places you can get your 2019 AGI:
If you didn't file a 2019 federal tax return (or you haven't filed one yet), enter 0 as your AGI.
Married Filing Separately in 2019 but filing jointly in 2020?
Filed jointly in 2019 but are Married Filing Separately in 2020?
You won't find your AGI on your W-2 or 1099 form because those forms don't take into account over a dozen above-the-line deductions that go into calculating your AGI.