overall step is :create msg -->compile msg--> show msg

1 make workspace
#wheeltec@wheeltec:~$ mkdir -p launch_sensor_ws/src
#wheeltec@wheeltec:~$ cd ~/launch_sensor_ws/src

2 create package

wheeltec@wheeltec:~/launch_sensor_ws/src$ catkin_create_pkg launch_sensor_pkg std_msgs message_generation roscpp rospy RPi


wheeltec@wheeltec:~/launch_sensor_ws/src$ cd launch_sensor_pkg

3 create msg

wheeltec@wheeltec:~/launch_sensor_ws/src/launch_sensor_pkg$ ls
CMakeLists.txt  include  package.xml  scripts  src
wheeltec@wheeltec:~/launch_sensor_ws/src/launch_sensor_pkg$ mkdir msg
wheeltec@wheeltec:~/launch_sensor_ws/src/launch_sensor_pkg$ cd msg
wheeltec@wheeltec:~/launch_sensor_ws/src/launch_sensor_pkg/msg$ vim sensorEnvMsg.msg


notice:data type must correct

wheeltec@wheeltec:~/launch_sensor_ws/src/launch_sensor_pkg/msg$ cat  sensorEnvMsg.msg
int32 msg_id
float32 temperature_value1
float32 atmosphere_value
float32 temperature_value2
float32 humidity_value
float32 gas_value
float32 rain_value
time msg_datetime



4 edit package.xml,remove remark !-- of two lines as follows  

wheeltec@wheeltec:~/launch_sensor_ws/src/launch_sensor_pkg$ sudo nano package.xml



  <!-- Use build_depend for packages you need at compile time: -->

     <build_depend>message_generation</build_depend>  
  
  <!-- Use build_export_depend for packages you need in order to build against this pac$
  <!--   <build_export_depend>message_generation</build_export_depend> -->
  <!-- Use buildtool_depend for build tool packages: -->
  <!--   <buildtool_depend>catkin</buildtool_depend> -->
  <!-- Use exec_depend for packages you need at runtime: -->

      <exec_depend>message_runtime</exec_depend>  

5 edit CMakeLists.txt 

wheeltec@wheeltec:~/launch_sensor_ws/src/launch_sensor_pkg$ sudo nano CMakeLists.txt 

5.1

## is used, also find other catkin packages
find_package(catkin REQUIRED COMPONENTS
  RPi
  message_generation
  roscpp
  rospy
  std_msgs
)


5.2

## Generate messages in the 'msg' folder
 add_message_files(
   FILES
   sensorEnvMsg.msg
#  Message2.msg
# )

5.3

## Generate added messages and services with any dependencies listed here
 generate_messages(
   DEPENDENCIES
   std_msgs
 )



5.4 must is #

catkin_package(
  INCLUDE_DIRS include
#  LIBRARIES launch_sensor_pkg
#   CATKIN_DEPENDS message_generation message_runtime  roscpp rospy std_msgs
#   DEPENDS system_lib
)


6 compile  

$ source devel/setup.bash 
wheeltec@wheeltec:~/launch_sensor_ws$ catkin_make


ok as follows

Scanning dependencies of target launch_sensor_pkg_generate_messages
[100%] Built target launch_sensor_pkg_generate_messages
wheeltec@wheeltec:~/launch_sensor_ws$ 


7 rosmsg show msgname

wheeltec@wheeltec:~$ rosmsg show sensorEnvMsg.msg

[launch_sensor_pkg/sensorEnvMsg]:
int32 msg_id
float32 temperature_value1
float32 atmosphere_value
float32 temperature_value2
float32 humidity_value
float32 gas_value
float32 rain_value
time msg_datetime