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.

It is pretty misleading that it says "nothing to pay" on that page when in practice it often just means nothing yet overdue. You can look up your balancing payment and first payment on account for next year via the file a return service, "View your calculation" then "View and print your full calculation". Note that down, and then follow the links "Tax return options", "View account", "Tax years" to get to a place where you can look up your payments on account already paid. With that information, you can calculate (balancing payment) + (first payment on account for next year) - (payments on account already made) to figure out exactly how much to pay before the end of January. It's a strange quirk of the system that it won't just tell you how much to pay, and makes you calculate it yourself.