Hi,
change the admin password and restart the service then you can able to put the server in manulal detention.Please follow the below steps.
open the command prompt/terminal of your system. Find the passwd file( $SPLUNK_HOME/etc/passwd ) of Splunk and rename it as passwd.bk.
mv /opt/splunk/etc/passwd /opt/splunk/etc/passwd.bk
Create a .conf file names user-seed.conf in your $SPLUNK_HOME/etc/system/local directory.
cd /opt/splunk/etc/system/local/user-seed.conf
[user_info]
USERNAME=admin
PASSWORD=<new_password>
/opt/splunk/bin/splunk restart