Hi Raiq,
The same thing happened to me earlier. We fixed the issue by modifiying the adgentns.pl and adconfig.pl.. we added the ora18in.jar in to the classpaths defined there in those perl scripts.. Those perls script execute java and we modified their classpath (added ora18in.jar) used for executing those java .. With this action, the issue is fixed! So we can continue at the moment..
I even wrote a blog post about it ->
https://ermanarslan.blogspot.com/2021/10/ebs-rdbms-19c-upgrade-post-upgrade.html1.) In adconfig.pl we added orai18n.jar path to classpath TO line 933
-->> $ENV{'CLASSPATH'} = $classpath.":/u01/app/oracle/product/19.0.0.0/dbhome_1/jlib/orai18n.jar";
2.) In adgentns.pl file we added following line (between 173 and 174 )
-->> $classpath=$classpath.":/u01/app/oracle/product/19.0.0.0/dbhome_1/jlib/orai18n.jar";