The above error message appears when you try to deploy a Windows Phone Project to a physical Windows Phone device.
Lock Screen is active on phone
Unlock the screen on your Windows Phone-based device
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This simple article explain how to install certificates file .cer on Nokia Symbian Phone.
Mobile devices not accept all certificate formats.
How to install certificates file .cer on Nokia Symbian-based devices
Rename .cer file in .crt then copy it the across to your device and run installation tapping on the file into the device.
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