Ok so you need to follow the tutorial. If you are getting that message it means you have not followed the tutorial and did not setup the app in SambaPOS correctly or did not give correct permissions. Or you are not running message server correctly.
You have to follow the instructions completely to get it to work. This means setting up the app inside Sambapos and giving correct permissions, setting up Message server to allow the light weight html server and for GQL.
Show us screenshots of SambaPOS where you setup the application mobilepos
Here is mine for example: