Indeed, ResearchGate allows to create DOIs only for unpublished research, see https://help.researchgate.net/hc/en-us/articles/14293044585745-ResearchGate-DOIs. For a possibility to create DOIs to publications see Rob Keller's reply to https://www.researchgate.net/post/Can_I_use_ResearchGate_DOI_for_an_article_uploaded_in_RG_which_is_published_elsewhere_in_a_Journal with a link to Zenodo providing DOIs. But note also that DOIs are identifiers for digital objects. A publication that is not available as a file (i.e., is printed only) cannot have a DOI.

A DOI (Digital Object Identifier) is a unique permanent web address (URL) with a string of numbers, letters, and symbols used to identify a specific digital object, such as a research paper or article. A DOI will always refer to that article, and only that one. Web addresses (URLs) might change, but DOIs will stay the same. Please note that only digital articles can have DOIs, and those avaialble only in prints cannot have a DOI !

There are registration agencies (RA) to assign DOIs. These agencies are members of the DOI Foundation, who govern the DOI system on behalf of the agencies who manage DOI registries and provide services to their respective clients (https://www.doi.org/ ). Crossref and DataCite are examples of registration agencies to assign DOIs.

Normally, publishers or researchers register with a chosen RA and pay a fee to obtain a DOI prefix. They then assign DOIs to their digital objects (e.g., articles) according to the RA's guidelines.

There is one more option as Wolfgang R. Dick mentioned. An article without an already assigned DOI can try for ResearchGate DOI (https://help.researchgate.net/hc/en-us/articles/14293044585745-ResearchGate-DOIs)

Thank you, C George Thomas, for these additions. But note that ResearchGate will not assign DOIs to articles. See https://help.researchgate.net/hc/en-us/articles/14293044585745-ResearchGate-DOIs: "ResearchGate DOIs can be generated for most of your unpublished work. As publications classified as article, book, chapter, patent, cover page, poster, and conference paper are considered to have been published elsewhere, ResearchGate DOIs cannot be generated for these types of research."

OK! Thank you for the clarification. ResearchGate DOIs are meant for unpublished work shared through ResearchGate. For already published articles elsewhere, one has to approch the registration agencies such as Crossref and DataCite