You can find the answers in here -

https://www.researchgate.net/post/Can_we_assign_DOI_number_for_an_already_published_journal_article

Here you will find many good answers:

https://www.researchgate.net/post/Generate_DOI

Thank you, Mr. Anton Vrdoljak.

You can find the answers in here -

https://www.researchgate.net/post/Can_we_assign_DOI_number_for_an_already_published_journal_article

Hi, I share this information of ResearchGate

Provided the type of research you have added supports adding a DOI, you can generate one by following these steps:

Go to your Research tab

Select the research item you would like to generate a DOI for by clicking on its title

On the right-hand side, click on the blue downward-facing arrow and select Generate a DOI (if this is not visible, then generating a DOI for this type of research item is not possible)

Review the details of your research item to ensure they are correct

Click Generate a DOI.

Note: Once a ResearchGate DOI has been generated for a research item, you’re no longer able to edit that research item. Instead, you should remove the research completely, re-upload it with the edits and generate a new DOI.

I agree with Rabin Thapa

On ReserachGate DOI is generated automatically for Preprints (If it is not generated then please follow the following steps)

1. Click on the research tab on your profile page.

2. Select the research work for which you want to generate DOI

3. On the right side you can find a bottom faced blue arrow, click it.

4. Thereafter you can find generate DOI.

Note - This is only possible for preprints. Whereas in the case of articles already published this is not possible.

Thank you.

Go to your Research tab. Select the research item you would like to generate a DOI for by clicking on its title. On the right-hand side, click on the blue downward-facing arrow and select Generate a DOI (if this is not visible, then generating a DOI for this type of research item is not possible)