F12 is used by Visual Studio debugger to trigger a breakpoint and apparently it cannot be changed:
"The F12 key is reserved for use by the debugger at all times, so it should not be registered as a hot key. Even when you are not debugging an application, F12 is reserved in case a kernel-mode debugger or a just-in-time debugger is resident."
Source: https://msdn.microsoft.com/en-us/library/windows/desktop/ms646309.aspx?f=255&MSPPError=-2147217396
MrSimbax