ResearchGate generates DOIs only for unpublished research, not for publications of the types "Article", "Book" or "Chapter", but, e.g., for the type "Thesis". See https://help.researchgate.net/hc/en-us/articles/14293044585745 for more information.
Dear Peter Walgemoed In addition to what is indicated by Wolfgang R. Dick : if you want to add a DOI to a paper you can use Zenodo https://zenodo.org If I recall correctly you need to register (for free) and then you can use this service.