In case, if you installed/upgraded new versions of jenkins and unable to find admin credentials on server then, ...
if you are using old version of jenkins and on the top of it you are trying to reinstall/upgrade new version of jenkins then,
the files under "JENKINS_HOME", namely -
- ${JENKINS_HOME}/jenkins.install.InstallUtil.lastExecVersion
- ${JENKINS_HOME}/jenkins.install.UpgradeWizard.state
will cause jenkins to skip the unlock (or admin credentials screen) and webpage directly ask you for username and password. even on server you will not able to find "${JENKINS_HOME}/secrets/initialAdminPassword".
In such case, don't get panic. just try to use old admin user creds in newly installed/upgraded jenkins server.
In simple language, if you have admin creds as admin/admin in old version of jenkins server then, after upgrading jenkins server, the new server won't ask you set password for admin user again. in fact it will use old creds only.