Kezzy KM

I completed my tax return in August 2023. I ticked the box to pay my tax through my employment PAYE code. I tried to call, I think in January as I could not see any confirmation and had read of others on the forum who had asked the same question. There was no one to take the call with the service currently closed due to tax return deadline. Today, I have come home to an 'overdue' letter, which understandably I am bemused and cross about due to my request on the form. HMRC are shut today! Can anyone offeer any advise why my tick box for PAYE through my employers code has been ignored. Thanks