I am following this answer https://answers.ros.org/question/323429/how-to-create-custom-action-interface-definition-in-ros2/ in order to create a pkg for a custom action. My system is Ubuntu 18.04 with ROS2 dashing. My package looks like this:

 action_interface
      action
            PlanAid.action
      CMakeLists.txt
      package.xml

My files are: PlanAid.action

#goal definition
string request
string problem
 ---
#result definition
string data
string event
---
#feedback
string feedback

CMakeLists.txt:

cmake_minimum_required(VERSION 3.5)

project(action_interface)

# Default to C++14
if(NOT CMAKE_CXX_STANDARD)
  set(CMAKE_CXX_STANDARD 14)
endif()
if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_CXX_COMPILER_ID MATCHES "Clang")
  # we dont use add_compile_options with pedantic in message packages
  # because the Python C extensions dont comply with it
  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Wpedantic")
endif()

find_package(ament_cmake REQUIRED)
find_package(rosidl_default_generators REQUIRED)
find_package(std_msgs REQUIRED)
find_package(action_msgs REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  action/PlanAid.action
  DEPENDENCIES std_msgs action_msgs
)

ament_export_dependencies(rosidl_default_runtime)
ament_package()

package.xml:

<?xml version="1.0"?>
<?xml-model href="http://download.ros.org/schema/package_format3.xsd" schematypens="http://www.w3.org/2001/XMLSchema"?>
<package format="3">
  <name>action_interface</name>
  <version>0.0.1</version>
  <description>ros action custom action interface</description>
  <maintainer email="skanda.naglapur-ramamurthy@man.eu">Skanda Naglapur Ramamurthy</maintainer>
  <license>MIT</license>

  <buildtool_depend>ament_cmake</buildtool_depend>
  <buildtool_depend>rosidl_default_generators</buildtool_depend>

  <build_depend>std_msgs</build_depend>
  <build_depend>action_msgs</build_depend>

  <exec_depend>rosidl_default_runtime</exec_depend>
  <exec_depend>std_msgs</exec_depend>
  <exec_depend>action_msgs</exec_depend>

  <member_of_group>rosidl_interface_packages</member_of_group>

  <export>
    <build_type>ament_cmake</build_type>
  </export>
</package>

And I am having the following error when colcon build --symlink-install

    --- stderr: action_interface                               
    /home/developer/task-manager/ros2_ws/build/action_interface/rosidl_generator_py/action_interface/action/_plan_aid_s.c:11:10: fatal error: numpy/ndarrayobject.h: No such file or directory
     #include "numpy/ndarrayobject.h"
              ^~~~~~~~~~~~~~~~~~~~~~~
    compilation terminated.
    make[2]: *** [CMakeFiles/action_interface__python.dir/rosidl_generator_py/action_interface/action/_plan_aid_s.c.o] Error 1
    make[1]: *** [CMakeFiles/action_interface__python.dir/all] Error 2
    make: *** [all] Error 2
    ---
    Failed   <<< action_interface   [ Exited with code 2 ]

    Summary: 2 packages finished [14.0s]
      1 package failed: action_interface
      1 package had stderr output: action_interface
      5 packages not processed

What is strange is that if I conda deactivate it compiles

--- stderr: action_interface                               
/home/developer/task-manager/ros2_ws/build/action_interface/rosidl_generator_py/action_interface/action/_plan_aid_s.c:11:10: fatal error: numpy/ndarrayobject.h: No such file or directory
 #include "numpy/ndarrayobject.h"
          ^~~~~~~~~~~~~~~~~~~~~~~
compilation terminated.
make[2]: *** [CMakeFiles/action_interface__python.dir/rosidl_generator_py/action_interface/action/_plan_aid_s.c.o] Error 1
make[1]: *** [CMakeFiles/action_interface__python.dir/all] Error 2
make: *** [all] Error 2
---
Failed   <<< action_interface   [ Exited with code 2 ]

Summary: 2 packages finished [14.0s]
  1 package failed: action_interface
  1 package had stderr output: action_interface
  5 packages not processed