Welcome to the Community and thanks for posting. Apologies you have not been able to update your payment details online, let me see if I can sort this out for you.
I have just tried to replicate this error in my own online account and it seemed to work just fine so I was unable to replicate this problem unfortunately. The method I used was removing the current card and then adding another card rather than using the option to update card details. You may want to try that method to start with if you've not already tried it.
It may also be worth trying to change the card details in another internet browser just in case there is something underlying in your particular browser.
If you still have these problems after trying these things can you also let me know at what point that error appears please? Is it something that pops up as soon as you click on the Payment card Tab or when you select an option within there?
Let me know how you get on after trying those things
On a different Windows 10 laptop Tried both Edge and IE11 with same result
Login using email address and password. On Account Hub page, click Top up On Top up page, click Payment card On Payment card page, click Add a payment card Choose a card, put dot in Use new card details and click Enter your card details
Bank system can't connection Please check and try again later