Companion Certificate terms say you can use it for a W fare (i.e. DeltaComfort). I am looking at a number of flights that has W available, but it is saying NOT AVAILABLE.

I've always used the reserve companion pass for either First (Z / I) or Coach since I get the DeltaComfort for free. But the First Class (Z, I) and Coach required inventories (L,U,T,X,V) are sold out.