Depending on the number of passengers, that could be a lot or a little. It is hard to force BA to issue a ticket at a price at which it would not issue a ticket even if it took a bit of time to sort why, but if it is base fare rather than taxes, that is something which BA at a managerial level might do as a customer service matter.

To answer the broader question, it does happen that the purchase cannot be completed and requires manual intervention. It could be because the fare is no longer available (all it takes is to have just one seat available at the price point and have someone else complete their purchase while you are in process). It could also be because there is an error.

If it is taxes, not to be callous, but too bad. If it is fare, it is worth a go.