It sounds as if you are trying to make a back door Roth IRA contribution.
You can still make a non-deductible contribution to a Traditional IRA for 2020 prior to April 15, 2021, as long as you have not reached the maximum IRA contribution limit for your age for 2020.
Then, the Traditional IRA may be converted to a Roth IRA any time after that. However, the conversion will be a 2021 conversion since it will occur during the 2021 calendar year. You would receive a Form 1099-R for 2021 that would be reported on your 2021 tax return. The only thing you would report on your 2020 tax return would be the non-deductible Traditional IRA contribution.
This would hold true for any conversion that is done during 2021 -- conversions are determined by when they are completed and there is no provision to be able to convert an IRA for a prior tax year up to the due date of the return.