Did this happen the first time you used the drives or did they operate a bit before you got the error message?
Note that the 4TB WD Purple NV WD4NPURX does not appear in the official published compatibility list for your device. I would contact the manufacturer of the NAS and ask specifically about this model if it can function in the NAS or not.
You can take the drives out of the enclosure and connect them to a working PC and see if both of them pass the quick and the extended tests of WD Data Lifeguard Diagnostic. This should show you if the drives function properly or not.
Post back with some info from the manufacturer and from the tests.