Can anyone help in fixing this constant "ModuleNotFoundError" on Google Colaboratory?

you need to download and install the library before you can use it. I saw that there is a github for your package here: https://github.com/dullemond/radmc3d-2.0

in colab, you can install it with:

!git clone https://github.com/dullemond/radmc3d-2.0.git
%cd ./radmc3d-2.0/python/radmc3dPy/
!python setup.py install
%cd ~/../content

! and % are required.

The comand clones the desired directory from git, changes into the python directory, runs the setup of your package and changes your working directory back. Directly running the setup.py with the full path, did not work, hence changing the directory.