As the error message indicates, the reason why the designer cannot be loaded could be an issue with an older version of Kendo UI Mobile used in your project. Please note the UI Designer tool was introduced an experimental module by this time.
In order to advice you further, would you share more detailed information on the exact case you have? What version Kendo UI Mobile do you have?
How do you test this? It will be very helpful if you can capture and provide a video illustrating the step-by-step actions you perform at you end.