Can I go back into my 2020 taxes to fix a mistake I made on the very same day I filed?

Sorry but once you hit the transmit now button you cannot make any changes at all until the IRS either accepts or rejects the return. If the return is rejected then you can make all the changes you want and resubmit the return. The IRS said they would start processing returns on February 12th so you will have to wait until they take your return for processing.

If the return is accepted, then you must wait until the original return is fully processed and you have the original refund in hand,  only at that point can you amend the return.  While you're waiting for the IRS do not make any changes in your return at all, doing so will mess things up  going forward... If you have made any changes undo them since you must start with the original return to either fix or amend.