Yes, you can use your 2019 earned income instead of your 2020 income to compute the Earned Income Tax Credit (EIC). Please see What are the qualifications for the Earned Income Credit (EIC or EITC)?
Please follow these steps in TurboTax:
When I followed those instructions, it's telling me that I can't e-file due to a federal issue with my 2020 income??
If you have no income on your 2020 return, it may be causing this error. Although a return can be e-filed for the Recovery Rebate Credit, for certain taxpayers who have only social security income or SSI, it's not the same for credits like the earned income tax credit (EITC). This credit undergoes more scrutiny.
For more information about what situations may cause this issue: When a tax return will not e-file