Payments on account/balancing payment due but says I owe nothing?

I am a PAYE employee but also a sole trader for a small side business. I completed my self-assessment for 2023-2024 in May 2024. My tax calculation said I owed a balancing payment of approx £12k for 2023-24, and that I will owe a payment on account in Jan 2025 of approx £6k and another payment of approx £6k in July 2025. So I owe HMRC about £18k to be paid by Jan 31st 2025. I made about £7k in payments on account in 2023-24 so subtracting those should mean I owe roughly £11k by Jan. I want to pay this now, but HMRC's self-assessment portal says that I have nothing to pay. How can this be? I want to avoid a situation in January where I get fined but it makes no sense to me. How can I make a balancing payment for last year when the SA portal says I owe nothing? I must do, surely? Is the balancing payment being taken off my PAYE earnings?

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Not HMRC...you can pay anytime, you dont need to wait for it to update. The balancing payment/first payment on a/c becoming due 31/1 dont usually update to show the payments as actually due/payable until around now/early December (HMRC have started to prepare statements showing these), been that way since 1997.