I tested what you said an the app was correctly signed after a build in the CLI. Here are the steps I followed:
1) Run appbuilder create hybrid WpTestApp
2) Run cd WpTestApp
3) Run appbuilder build wp8
4) After the build was completed, I scanned the first produced QR code and installed the app successfully on a Windows Phone device (Microsoft Lumia 550).
If you get a message, which says that the company app can't be installed, it means that you didn't install the Application Enrollment Token (AET) provided by AppBuilder first. When you make a successful build in the CLI, you get a page with two QR codes (see screenshot in attachment). If this is the first app from AppBuilder that you install on this device, you need to scan the second QR code first. After the AET is installed, you shouldn't have problems with getting the app installed.
If the issue persists, please open a support ticket and send us the XAP file that you have problems with.
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