The IRS has stated that the payments for 2020 are also due when the tax return is due. There should be no penalty for paying owed taxes after April 15 as long as they are paid by the May due date for tax returns. I am not aware that they have said anything about the estimated tax payments due quarterly. Estimated tax payments would not be expected to be an issue as long as the total for the tax year is paid when due. So if your estimated tax is for 2021, there should not be a penalty. IRS.gov does not say anything about this or at least it is not found in a search.
