We were a few months into 2020 when Congress passed a law waiving the RMD requirement for the year, so Congress gave taxpayers until August 31, 2020 to return an early-year RMD distribution to an IRA or other retirement account. Report this as a rollover, even if it does not satisfy the usual 60-day rule for rollovers. Also note that it does not count as a rollover for purposes of the rule restricting you to one rollover per year. Whatever software you use to prepare your return should be able to handle this if you specify the distribution was rolled over.
As an aside, in some situations IRS rules require the designation “taxable amount not determined,” even if the reporting entity knows the taxable amount is zero. It’s a way of categorizing the transaction, not an indication that Fidelity is actually unclear about the tax treatment.