I have the same problem with a book recently uploaded, I can't generate the DOI for it using the traditional procedure in ResearchGate. I hope that this feature has not vanished.
I haven't found any way or method to create the DOI in ResearchGate, just found this question posted in RG, one year ago, but there is no more information about it. May be you have already seen the post.
"Pour les membres de la communauté de l’Enseignement Supérieur et de la Recherche française, le coût se limite à une cotisation annuelle de 180 € quel que soit le nombre de DOI. Pour tous les autres usagers, le coût correspond à une cotisation annuelle de 180 € pour l’attribution d’un maximum de 500 DOI, un tarif dégressif étant appliqué au-delà."
So, it's not free of charges. You have to subscribe for.
It appears that only certain types of documents are elgibile for a RG DOI. Conference Papers are one of the types that seem to be excluded. Posters, however, is a supported doc type.
Would certainly be good if RG could provide some info on what it generates DOIs for.