Absolute and Relative paths

edit

You need to share your CMake code for people to be able to help you.

Apologies. Here is the CMake file.

cmake_minimum_required(VERSION 3.14)
project(ros2_msgs)

SET(ENV{PYTHONPATH} "/opt/ros/dashing/lib/python3.6/site-packages/")
SET(CMAKE_PREFIX_PATH "/opt/ros/dashing")

find_package(rosidl_default_generators REQUIRED)

rosidl_generate_interfaces(${PROJECT_NAME}
  "${CMAKE_CURRENT_SOURCE_DIR}/srv/Connect"
)

Reason for setting the prefix path and the python path is because I am trying to build without using colcon,

The prefix path (CMAKE_PREFIX_PATH) has nothing to do with the error message. But you shouldn't put that into your CMakeLists.txt but instead set it in the environment before building your package - otherwise you can't build your package with other ROS distros - or in combination with a from-source build on other platforms.