"I noticed in the IRS instructions for 1040, it says to put zero in box 2a if you rolled over the entire amount in 2020."
Nowhere do the IRS instructions for either Form 1040 or Form 1099-R say that.
A code 7 Form 1099-R for a distribution from a traditional IRA must always have the same amount in box 2a as is in box 1 and the Form 1099-R must be entered exactly as received. Since the entire amount was rolled over, you must indicate that none of the distribution was RMD (no distribution in 2020 was an RMD because RMDs for 2020 were suspended), indicate that you moved the money to another retirement account or back to the same account, then indicate the amount that you rolled over. TurboTax will include the entire gross amount on Form 1040 line 4a but will exclude from line 4b the amount rolled over. TurboTax will include the word ROLLOVER next to the line. If you don't get this result, delete and reenter the form 1099-R, making sure to indicate the none of the distribution was RMD.
