If your customers are receiving a "Currency not supported by merchant" response when they try to pay you, it means you are trying to charge them in a currency that your business on Paystack is not enabled for.
By default, your business is enabled for the official currency of your country. For example, NGN is the default currency for Nigerian businesses and GHS is the default currency for Ghanaian businesses.
If you will like to process payments in other currencies, you will need to ensure the currency is available for your business, and then make a request for it to be activated. Please see What currency is available to my business? for more information on the currencies available to your business.