If you pay for a transaction by a debit or card, and there are problems with it, you may be entitled to a chargeback, which is “reversal of a disputed sales transaction on a credit or debit card. … The card provider will decide if you are entitled to a refund based on the circumstances. …”. Assume you booked a flight some time ago, but, on the day of the flight, the destination was one in respect of which Government guidance was, for Covid reasons, that you should not travel. Let us further assume that the flight nevertheless operated, so that it was not cancelled, and the airline refused to refund your ticket price. In those circumstances, you could seek your money back via chargeback from your card provider. In most such circumstances, card providers do indeed decide to make the refund.
Now, let us assume that the airline in question is Ryanair. Article 7 of their Terms and Conditions deals with circumstances where they refuse to carry a passenger; it provides, in part:
Article 7 – Refusing to carry a passenger
We may refuse to carry you or your baggage on any flights operated by an airline of the Ryanair Group, if one or more of the following circumstances apply, or we have good reason to believe that they may apply.