A currency mismatch error occur when your platform (as a merchant) has a different currency from the supplier you have activated.
Even if was relatively simple for us convert prices from the supplier currency to any other currency you could have selected, there was no way to force the supplier to handle merchant's orders in another currency.
For this reason, to work together, suppliers and merchants must have the same currency.
As merchant, you can change your platform's currency usually from settings.
Each ecommerce platform is different but there are numerous guides online that describe this process.