If Nexus is requesting for license to be installed every time it is restarted, then it is most probably due to the OS user that Nexus is started as, has no home directory defined.
The license details is stored in java preference store. In Linux this would be $USER_HOME/.java/.userPrefs/com/sonatype/nexus/professional/prefs.xml.
There are two options to resolve the issue:
Option 1) Create a home directory for the Nexus process user (ensure that Nexus process user can write to the home directory).
Option 2) Add a property java.util.prefs.userRoot to the file $NEXUS_HOME/bin/nexus.vmoptions to define the location to store the java preference store (ensure that the directory exists, and the Nexus process user can write to this directory).
To do this, edit the nexus.vmoptions file and add a line like this:
Nexus should then be restarted and license will need to be installed, which will then be stored in the java preference store. This should then prevent license install request on subsequent restarts.