Newer versions of Java (Java 11+) may fail to run the legacy v2.3 jar. Try using Java 8, or ensure you are running it with java -jar ubnt-discovery-v2.3.jar from a command prompt to see errors.
The UniFi Controller software includes built-in discovery mechanisms. ubnt discovery 2.3 download software
Required to run the .jar file.
Force execution using an explicit path to a Java 8 binary: C:\Path\To\Java8\bin\java.exe -jar ubnt-discovery-v2.3.jar . Modern and Supported Alternatives Newer versions of Java (Java 11+) may fail