You don't need to adjust the cost basis assuming the cost basis was correct when you sold the RSUs. You are correct in that the value of the RSUs was already included on your W-2 when the RSUs vested. Your company sold some RSUs to cover the tax withholding, sometimes referred to as a "sell to cover."
There are times when the RSU cost basis does need to be adjusted. For example, if your 1099-B that included the sale of your RSUs reflected a cost basis of zero--and this is not uncommon because brokerage firms do not typically know the cost basis so they will assign a zero cost basis--then you do need to adjust the cost basis to reflect the value of the RSUs on the day they vested.
Yes, you can amend your return for tax year 2020. Before you amend your 2020 return, review your 2020 return and specifically Form 8949. Form 8949 is where you should see the sales of the RSUs. If you have a Form 8949, check to see what cost basis has been entered for the RSU sales. If it is zero, or some other number that is not correct, then it may be appropriate to amend your return.
Here is a link to the TurboTax webpage that contains the TurboTax products for 2020.
TurboTax 2020
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