Tomcat users are defined in the file
$TOMCAT_HOME/conf/tomcat-users.xml, by default, there is NO user, it means no one can access the Tomcat manager page.
To enable users to access the Tomcat manager page, add a user as the role
Step 0. Shutdown Tomcat if it is running. Stop Java process as well, just to be safe.
Step 1. Open tomcat-users.xml file, originally it should contain a commented out part for tomcat-users.