We did not receive our second-round stimulus payment until March, 2021. Do we still need to report it on our 2020 taxes?

Yes, you have to report both stimulus payments on your 2020 tax return, even if your receive the second payment in March 2021.

In fact, both these payments are advanced payments of the 2020 Recovery Rebate Credit.

Please read this IRS document on the Recovery Rebate credit.