Because pay-at-pump machines charge a final amount a few days after the first transaction, your account could be charged an amount that’s more than your current balance.
This could result in your balance being reduced to a negative figure, which is called an unauthorised overdraft. You would then be unable to use your card until the negative balance was cleared. 
We want to make sure that you’re always able to use your card, so we may decline pay-at-pump transactions if there isn’t enough money in your account to cover the potential cost.
I hope that makes things a bit clearer 
Please don’t ask us to do that 
COps find it much easier to diagnose issues when you give them a clear description of what is going on, and it will be much quicker for you too! 