I'm sure we can get to the bottom of this, I'll just need to ask you some troubleshooting questions first!
Can I ask what kind of errors you are seeing, or which part of the process you were finding difficult? If there was an error, did you see a banner or a pop up letting you know what the error was about? This will help me gather some context on what the issue is so I can help you resolve it! We do have a guide here on how to add a third-party payment gateway, and another guide here on setting up Shopify Payments, could you let me know the name of the gateway you're adding?
Also, can I ask what country you are currently residing in? Some payment gateways are only available to merchants who live in certain countries, so I'm wondering if you're trying to add a gateway that is currently not available in your location. You can check this out by selecting your country from this list of available payment gateways, this will show you which gateways are available to you. For example, Shopify Payments is only available in these countries at the moment.
Let me know the answer to the questions above and we can take things from there!
Ava | Social Care @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog