If you deferred paying your July 2020 Payment on Account, you will need to pay the deferred amount, in addition to any balancing payment and first 2020/21 Payment on Account, by 31 January 2021. This may be a larger payment than you usually pay in January.
If you are unable to pay your Self-Assessment bill in full by 31 January 2021, you can set up a Time to Pay payment plan of up to 12 months online without speaking to HMRC.