Thanks for reaching out here. I see this is your first post also, welcome to the Shopify Community.
I appreciate the screenshot - I can see that you are experiencing an error in your checkout when trying to place an order on your store.
From what I can see, you are using a third-party gateway called Mondido, is that correct? Have you confirmed that the setup for this gateway has been completed? You can check this in the Settings > Payments section of your admin. I would recommend clicking Manage beside the gateway, and making sure all of your credentials are correct, and that there are no obvious errors or warning messages relating to this gateway.
You can also log in to your specific Mondido merchant account if you have one, and double check that your account is valid and in good standing. For your information, we have some resources explaining how to set up third-party payment gateways on Shopify here.
You mentioned that you tried with test mode and the error still appears - can you confirm if you have tried this with test mode both off and on, and you are still seeing the same error both times?
Something else you can check for more specific error information is your abandoned checkouts. By navigating to Orders > Abandoned Checkouts you can click into one of these specific failed checkouts and see detailed payment events, which may provide you with additional information on exactly why the payment failed.
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