Hello Nawar A. Jasim

Please follow the link below.

https://www.crossref.org/guestquery/#textsearch

Follow the instructions provided in the link above to search for a DOI using the article title and author's last name. If this method doesn't yield a DOI, don't assume that a DOI doesn't exist. Follow up with the next two options provided in the link.

Please note, that some articles won't have a DOI. The International DOI Foundation was created in 1998 but not all publishers immediately started assigning DOIs. So unless a publisher has retrospectively assigned DOIs to articles, articles published prior to 1998 are less likely to have DOIs.

So, if you have tried all the ways provided in the link which I have mentioned above, but you still cannot find the DOI, it may be that your article does not have one.

Best.