You may have been one of the businesses that have struggled in the last 18 months with cashflow and taken advantage of HMRC's time to pay arrangements. The Business Payment Support Service has dealt with requests to defer over £4billion in tax so far.
What HMRC haven't managed to do is reflect those arrangements within the PAYE/NI underpayment notices issued over the last few weeks.
This means that you may well have had a demand for PAYE/NI which is already under an arrangement to pay with HMRC.
If you find yourself in this position you should contact HMRC on the number given on the Notice and tell them about your arrangements.