This indicates that you did not implement any Java code besides adding a Java action in Mendix:
com.mendix.systemwideinterfaces.MendixRuntimeException: Java action was not implemented
at MyFirstModule.Microflow_javaaction (JavaAction : 'Java_action')
Did you export your project to Eclipse and add your custom Java code already?
As Thorsten says, the default code in a newly generated Java Action is to throw the exception you are seeing.
To fix this, you need to go into the Java code that has been generated in Eclipse / VS Code / Notepad / your editor of choice, remove the throw exception line, and add your own code in.
More details on how to do this can be found in the Mendix documentation. https://docs.mendix.com/howto/logic-business-rules/extending-your-application-with-custom-java