"I opened a backdoor Roth IRA account last year (Dec 2019)"
I assume that this means that you did the Roth conversion in 2019, not in 2020. Since you also had another traditional IRA at the end of 2019 (the rollover IRA), the value of that IRA was required to be included in the calculation of the nontaxable and taxable amounts of your 2019 Roth IRA conversion. As such, it's likely that a large part of that conversion was taxable and a large portion of your nondeductible traditional IRA contribution made for 2019 remains with your traditional IRAs. Since your 2019 Form 8606 apparently shows the entire amount converted as nontaxable, you must amend your 2019 tax return to make the correction. You can do this by amending your 2019 tax return in TurboTax, clicking the Continue button on the Your 1099-R Entries page, and, when asked, enter the December 31, 2019 value of your rollover IRA. TurboTax will prepare the corrected Form 8606 and Form 1040X which you must file, and pay the balance due. (If your 2019 tax return did not include Form 8606 because you failed to report the contribution and Roth conversion, you must also make those entries when amending.)
You are not permitted to roll any basis in nondeductible traditional IRA contributions into a 401(k). Since your traditional IRAs presently do have some amount of this basis, you are not permitted to roll your entire rollover IRA into the 401(k). You are only permitted to roll over to the 401(k) the amount that is in excess of your basis in nondeductible traditional IRA contributions, determined as if all of the transactions occur on December 31, 2020. For this reason, you'll probably want to convert an amount equal to your total basis in nondeductible traditional IRA contributions before rolling the remainder (which will all be pre-tax money) over to your 401(k) before the end of 2020. You can do this after making your nondeductible traditional IRA contribution for 2020, as long as you make the contribution for 2020 before the end of 2020.
