The Order Not Found error message will be displayed on the Thank You page is in one case.
If the flow is not in the Test Mode (i.e it is live) and you open the thank you page directly without actually placing the order then this message will be displayed.
When, the Thank You page does not receive any order ID nor the session ID then it fails to pull the order information.
A better way to view the Thank You page just to verify the design. You can check the same by placing the flow in the Test Mode ( How to set Test Mode in CartFlows? ) and then you can view the Thank You page without any errors.
Note: Always remember to remove your flow from the Test Mode before making the flow live.