DOIs are not mandatory. If your publication does not have a DOI, you may simply leave the corresponding field empty.
If you really want to have a DOI, see the reply by Rob Keller to https://www.researchgate.net/post/What_is_DOI_and_How_to_create. ResearchGate provides DOIs only for unpublished research, see https://help.researchgate.net/hc/en-us/articles/14293044585745-ResearchGate-DOIs.
Getting a DOI (Digital Object Identifier) for your article involves a few steps, but it's a relatively straightforward process. Here's a general guide:
Publish Your Article: First, your article needs to be published in a journal or a conference proceedings that assigns DOIs to published articles. Most reputable journals and conferences do this as part of their publishing process.
Check with Your Publisher: If you've already published your article, check with the publisher to see if they have assigned a DOI to your article. They will usually provide it along with the other publication details.
Register Your Article with CrossRef: CrossRef is a non-profit organization that provides DOIs for scholarly content. Many publishers work with CrossRef to assign DOIs to their articles. If your publisher hasn't already assigned a DOI, you can register your article directly with CrossRef. You typically do this by filling out a form on their website and paying a fee if you're not a member.
Receive Your DOI: Once your article is registered with CrossRef, you'll receive a DOI for your article. This will typically be in the form of a URL that looks something like "https://doi.org/10.1234/example".
Include the DOI in Your Publication: Once you have the DOI for your article, you can include it in your publication along with the other citation details. This will allow readers to easily find and access your article online.
If you're unsure about any step in this process, you can always reach out to the journal or conference organizers for guidance. They should be able to help you through the process of obtaining a DOI for your article.