I'm a non filer, but I filed my first stimulus I got in 2020, to be able to get my $600.00 second stimulus

You cannot use the current online program to get the 2nd stimulus check.    The current online program is only for 2021 returns.

The only way to get that 2nd stimulus of $600 is to file a 2020 tax return and use the recovery rebate credit to get it as a refund on a 2020 tax return.   There is not a "non -filer" portal for the 2nd stimulus.   And in order to get the 3rd one you need to file a 2021 tax return and use the recovery rebate credit.   The credit shows up on line 30 of your Form 1040.

Wish you had posted sooner, since it is now too late to use online software for free to file a 2020 return.   But you can file on the paper form and mail it in.   It will take quite a few months for the IRS to process it.

2020

https://www.irs.gov/instructions/i1040gi

https://www.irs.gov/pub/irs-prior/f1040--2020.pdf

https://www.irs.gov/pub/irs-prior/f1040s1--2020.pdf

https://www.irs.gov/pub/irs-prior/f1040s2--2020.pdf

https://www.irs.gov/pub/irs-prior/f1040s3--2020.pdf

https://www.irs.gov/filing/where-to-file-paper-tax-returns-with-or-without-a-payment