The expert answer above is kind of long and I found it hard to follow. And I think there's a typo in it.
Both your earned income and AGI have to be less than the table. See, EIC Qualifications
https://ttlc.intuit.com/community/credits-and-deductions/help/what-are-the-qualifications-for-the-ea...
If you had a lower Income last year you can use 2019 income. The special lookback rule will allow lower income individuals to use their earned income from 2019 to determine their Earned Income Tax Credit and the refundable portion of the Child Tax Credit in 2020, since their lower 2020 income could reduce the amount they are eligible for.
You can look back and use your 2019 income for the EIC credit instead of 2020. See this post https://ttlc.intuit.com/community/taxes/discussion/i-m-at-a-section-asking-me-do-you-want-to-use-las...
