This message is shown when the created Java action does not include any code. Did you create the Java action yourself? What are you trying to do within the AfterStartup? Retrieving json from a resource can also be done by using an import mapping. You don't need a Java action to accomplish this.
As Ruud says this is the default error message you get when you have created a Java Action, but not added any code into it.
To fix this, you need to either remove it from your After Startup microflow, or implement some Java logic in the action.
To implement the java logic you will need to edit your action. You should be able to find the Java source code in your project’s javasource/<yourmodulename>/actions/<youractionname>.java file. Inside there, you will need to remove the exception between // BEGIN USER CODE and // END USER CODE, and replace this with your own logic.
Good luck!