This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
Are you getting the exact error message as above? Does your code look exactly like Armondo's? Since he posted this three years ago he probably isn't coming back to help and I don't want to spend my time debugging his problem if your problem is different.
As for Armondo's situation, the error states that the return type "json" is unknown:
Caused by: The Result type [json] which is defined in the Result annotation on the class [class struts2.actions.JsonSample] or determined by the file extension or is the default result type for the PackageConfig of the action, could not be found as a result-type defined for the Struts/XWork package [struts2.actions#convention-default#]
His configuration is wrong somewhere. What does your code look like?