I just got that error and solved it :). It's very cryptic for sure. Basically ROS generates .h files for your .action file in catkin_ws/devel/include/ folder. So first check that these files exists. Every type declared in these .h are wrapped in namespace "your_node". So in your action server class, all the types you are referencing from above .h file must also use your_node:: prefix. If you are not using this prefix OR if your class is in other namespace then you will see error like above.

Another reason could be that your package /include folder has another subfolder "your_node". So now this causes two include/your_node folders (one in devel and another in your package). This might confuse compiler. So I also made sure my package's include folder doesn't have subfolder named "your_node".