@monirehbastami, I see you are using mEDRA. Does the problem still occurs when you try to export an article DOi when using the test environment? Could you please also try it again and send me the last XML file from your tmp/ folder in your files folder on the server? For you (i.e. for mEDRA users), this thread might be also interested: Problem with DOI [OJS 3.4.0.4.] - #5 by Almar93.