2020/21 income tax, CGT, class 2 and 4 NIC liabilities normally need to be paid by 31 January 2022. However, HMRC have recently announced that provided the tax is paid by 1 April 2022, there will be no penalty, although interest accrues from 1 February 2022 at 2.75%.
If you need longer to pay, then you will need to agree a payment plan with HMRC.
