Thanks for posting in the Shopify Community, I'm happy to help you out.
If the error message you're receiving at checkout is This store can't accept real orders or real payments, then one of the following is likely the cause:
The shop's payment gateway is in test mode.
The shop's plan does not allow payment processing.
The shop is using a third party payment gateway that is triggering this error.
If using Paypal and customers are using a currency that is not supported by Paypal, then this would also hinder them from being able to checkout using this gateway. You can cross check your default currency with Paypal Supported Currencies documentation.
I recommend logging into your admin and navigating to Settings > Payments, and verifying whether your payment gateway is in test mode. If you're using Shopify Payments, you can find the steps to do so in this help document. If you're using a third-party payment gateway, I recommend contacting them directly so they can investigate further.
If the issue persists, just let me know and I can look into other troubleshooting steps. Cheers!
Greta | 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