The PVC is plastic, the valve is brass. You can't glue brass to plastic. This is why you need an adaptor - 1/2" PVC to 1/2" NPT (national pipe thread).
Cut off the PVC before the coupling. Take that and the valve to a big box store. Get a short length of the PVC, a coupling, FPT adaptor, teflon tape, tube of pipe dope, a valve, primer, and glue for the PVC. PVC cuts with a hack saw, file/sand the cut.
See what kind of valves they have - you may have to get an MPT (Male) / FPT (Female) adaptor.