You can convert it to a Roth IRA at any time. If the conversion is done in April of 2021, then you will receive a Form 1099-R for 2021 that will be reported on your 2021 tax return that you will prepare in early 2022.
Since the non-deductible Traditional IRA contribution was made for 2020, be sure that it is being reported on your 2020 tax return.
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