If your self assessment tax bill for a year is for £1,000 or more and you paid 80% or less of your total income tax liability at source, e.g. through PAYE, you’re required to make payments on account (POAs) to HMRC towards your next tax bill. However, due to a problem with HMRC’s computer you might be off the hook for the POAs for 2018/19.
If your SA tax bill for 2017/18 meant you should have been liable to POAs for 2018/19 (normally due on 31 January 2019 and 31 July 2019) but HMRC hasn’t demanded or included them in your SA account, you won’t have to pay anything until 31 January 2020. Because the error was HMRC’s they won’t charge you interest for paying the tax later than the usual due dates.