Hi there @OLB!
Don here from Shopify.
Could you provide us with a lot more information and context around the issue as you are experiencing it so we can try to understand and help out?
It sounds like the issue may be around your checkout, so if you can confirm the payment gateway you are using when this happens as well as exactly what you are doing and what you see on the screen (screenshots would be ideal) we can start to try to understand the situation and help out.
It would also be helpful if you can share a link to your store here, so we can try to follow the info you give here.