Amended 2018 return and got refund (state and fed) in 2020. How/or do I, note that this year?

Are you looking for your AGI to use on your 2020 tax return, to file electronically? That information would need to come from your 2019 tax return. 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. If you tried to enter "0" and were not successful at processing your return, you can ​​​​see the information here for steps to correct your AGI. If you can wait a few days then resubmit your tax return and see if you are successful at that time. If it doesn't work you will need to mail in your return.