NeoLoad embeds a JAR file related to HTTP/2. It is referenced in the <install_dir>\conf\agent.properties. This warning means that the JAR cannot be loaded either because the referenced path has been modified or the JAR files is not there anymore.

In which NeoLoad version do you have that message? Is it an upgrade from an old NeoLoad version? If yes which one?

There was a bug in the upgrade of load generators where the wrong JAR file was still referenced in the agent.properties file.

You can either re-install 6.1.2 into another folder to start from scratch or  you can look at the agent.properties file and see if the right alpn-boot.jar is referenced. For that look at the <install_dir>\lib folder since you may have both the old and new jar file.