Hello Robert-Paul,
Progress Telerik Platform does not officially support Windows 10 Mobile development. You can use AppBuilder to develop Windows 8.0/8.1 applications which can be deployed on Windows 10 Mobile devices, however, you cannot develop UWP apps. We have considered adding Windows 10 Mobile development support several times in the past, however, the customer demand for UWP and it is not high enough to justify the engineering resources required to implement it.
Regards,
Garo
Progress Telerik