As stated in the other answer, debit card payments even contactless ones, do not debit instantly or anywhere near instantly. They can take several days or even longer.
However, the fact that the payment was approved would indicate that the contactless device used by the retailer managed to connect to your bank account and could see there were sufficient funds. At this time the payment should also have "reserved" the funds so it will be "pending" as such. Your online banking may not show all types of pending transactions, it may only show Direct Debits for example, if you ring the bank I expect they will be able to see this payment and advise accordingly, out of interest who do you bank with?
If you can see your "available" funds you should find that includes this payment having been "reserved" ie you can't use that money, but if it's a small amount you may struggle to spot it. Again, this will vary by bank and how their internet banking system has been implemented.
I would expect this payment to debit your account the working day after you made it, or at worst the day after that. In theory with "faster payment" technology it could happened within 2 hours but not all banks implemented that system in the same way so delays can and do occur. If the retailer/merchant banks with the same bank you do, in theory the transaction could be instantaneous, again depending on the bank.
Short answer short, the transaction is fine, it won't be void, you just need to wait, probably a day or two.
This link has more info, especially useful if you bank with NatWest (as I say the technology is likely to vary by bank to some extent). Your own bank should have their own version of this.