To the best of my knowledge, before using credit cards on exchanges you must perform KYC and pass it as well, without it, you can't trade on most exchanges or you will be limited on some features of the exchange. So I want to ask, have you performed and pass the KYC? If yes, then it could be that they doesn't support your location. Also, the best way to know what happened is by contacting their customers service executives.
Nevertheless, there are other exchanges like Binance where you can use your card to purchase cryptocurrencies, but however you must pass their KYC and depending on the coin you want to buy (fiat), your country must be supported, that is, only few fiat currencies are supported by Binance.