If you're sure you've gone through the payment process correctly, then I think you'd better contact them about it:
https://www.scotlandspeople.gov.uk/contact-usMaybe try the usual - log out, clear the cache etc. or use a different browser., use a different card.....
I often have to Log Out and Log In again before the payment I have just made appears up at the top right side. I don't know if this is usual with other members of 'ScotlandsPeople'
Doddsie - it always seems to take a minute or so to register the payment in the no. of credits box.