CMake Error: The following variables are used in this project but they are set to NOTFOUND.

edit

From the error message, I don't believe this has anything to do with ROS_LANG_DISABLE.

Please set them or make sure they are set and tested correctly in the CMake files: Modbus_LIBRARIES

it would appear that some package in your workspace needs Modbus functionality and the CMakeLists.txt is probably using something like find_package(..) or find_library(..) and fails. Those then set Modbus_LIBRARIES to NOTFOUND, leading to CMake printing the error that you show.

You'll have to check what uses Modbus in your workspace and resolve any problem it has finding the required libraries.

Edit:

I'm going through the tutorial modbus to install it

Judging from the CMakeLists.txt you show, I doubt the modbus package you link to is the one that your package is looking for, for two reasons:

1. that package is a Python package (so does not provide any C++ libraries)
2. your CMakeLists.txt uses find_library(..) and find_path(..), instead of find_package(..)

You'll have to figure out which particular library/package/project your package requires, otherwise this will be difficult to fix.

Is there any information in a readme perhaps, or some other form of documentation? Does the package.xml of your package tell you anything?

but I'm an absolute beginner with ROS

please note: this is not actually a ROS problem (any more), but a general "how do I make sure all dependencies of my software are installed" and CMake problem. When looking for help, you don't need to limit yourself to ROS fora.