Hi @Surojit_Bera,
Here is posts of users encountering similar errors here and addressing them here (though verifying their crossref credentials): CrossRef XML Explort Plugin - DOI Submission error - #2 by bozana
Perhaps you could try that and let me know if it works?
And, just a note, that the version of OJS that you’re using (3.1) is no longer supported by PKP. I recommend that you upgrade to the newest version of OJS, as there are some improvements in OJS and the Crossref plugins that you may wish to take advantage of. Along with that, you may wish to upgrade your Crossref plugin.
Upgrading instructions are available in the PKP Administrator’s Guide .
Information about the latest version of OJS can be found on the [PKP Website]https://pkp.sfu.ca/ojs/
The Crossref documentation can be found here: Crossref OJS 3.2+ Manual
-Roger
PKP Team