Your System doesn't support "Web Bluetooth" or your PC doesn't
have a active Bluetooth interface. Try to use the Chrome browser and activate your Bluetooth.
Old operating systems (Windows 7, ...) are not supported!
To check web-bluetooth compatibility visit WebBluetoothCG.
Also check the chrome flags and type chrome://flags in the address bar. You need to enable Experimental Web Platform Features.
You can find minimal examples to test here