Usually, DOIs are assigned by the journals prior to publication and are published together with the paper. However, some journals do not want to spend money on this, so that no DOIs are assigned. When there is no DOI at the time of publication, you will not get one later. As DOIs are not mandatory, you may not worry about this. If you wish to get a DOI anyway, see my reply to https://www.researchgate.net/post/How_to_get_a_DOI_here.

Thank you sir

Dear Rajesh SIVAKUMAR Sivakumar

Firstly - congratulations on getting your paper published in a Scopus-indexed journal!))) A DOI (Digital Object Identifier) is indeed essential for your paper. It is a unique alphanumeric string assigned to a digital object, such as an academic article, and serves as a permanent link to that object on the internet. DOIs make it easier for readers to find and cite your work, and they also ensure that your paper remains accessible and citable over time.

In terms of when you'll receive the DOI for your paper, it typically depends on the publishing process of the journal. Some journals assign DOIs shortly after a paper is accepted and goes through the final stages of production, while others may assign DOIs upon publication or shortly thereafter. It's best to check with the editorial office or the publisher of the journal for specific information about when you can expect to receive the DOI for your paper.

If your paper has already been published and you haven't received the DOI yet, I recommend reaching out to the editorial office or the publisher to inquire about the status of the DOI assignment. They should be able to provide you with more information and assistance.

Once you have the DOI for your paper, be sure to include it in your publication details, such as in the header or footer of your paper, so that readers can easily find and cite your work.

Sincerely