Sophia here from Shopify. Welcome to the Shopify Community!
I'm happy to help you out with this, though I'm not currently able to access your store through the sxnw.eu domain. This is the error message I'm running into with the
https:// results in this privacy error message:
This may mean that the DNS settings for the domain need to be adjusted. If it was just connected to your store recently and the settings are correct, it may just need 24-48 hours to fully connect. Let me know if you need any help with the domain.
In the meantime, can you tell me a bit more about how you're testing the checkout and what isn't working? If there are specific payment providers involved or error messages coming up, please let me know and share screenshots here if possible. Thank you!