Hi, Dušan!
My name is Ava, I’m one of Shopify’s Gurus and I’m here to help :)
You would need to use a payment gateway that has an integration and with your countries financial institutions in order to accept credit card payments from your customers at the checkout. When you find a suitable gateway, you can set up an account with the payment gateway on their website. Once that account is created, you can add that payment gateway to your Shopify admin, creating a link between the accounts.
Could you make sure you have your account set up correctly with your payment gateway?
Alternatively, the error message you are seeing may be because this gateway is not included on the list of available services for your country. To check this out, you can visit www.shopify.com/payment-gateways, then add /your-county to the end of the URL! So for example, if you lived in Sweden it would be www.shopify.com/payment-gateways/sweden, or if you lived in Czech Republic it would be www.shopify.com/payment-gateways/czech-republic.
The list of gateways you will see there are the only ones you could use for your Shopify store. This is because Shopify has an agreement between these gateways and the financial institutions/banks of certain countries to accept payments in certain currencies. So some payment gateways are not available in certain countries.
If you have double-checked that your payment gateway account is set up correctly, and you are using a gateway available in your country, but are still seeing this error, I'd like to look into this further for you.
Would you be able to send us an email to support@shopify.com using the email address listed as the account owners? Once you have done that, you can pop back here and let me know the support ticket number you were assigned (which will be emailed to you). That way, I can pick the ticket out of the queue and take a closer look at your settings!
Let me know how you get on :)
Ava
Social Care Guru | support@shopify.com