Problem with installing ROS on Ubuntu 18.04

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I am trying to install ROS on a Ubuntu 18.04, I have followed the steps from the wiki for installation and cannot complete this following line:

sudo apt install ros-melodic-desktop-full

The error message prints out:

Reading package lists… Finished
Builds dependency trees
Reading state information ... Finished
Some packages could not be installed. It may mean that you have requested
an impossible situation or, if you use the unstable distribution
that some necessary packages have not yet been created or moved
from "Incoming".
The following information may help to resolve the situation:

The following packages have dependencies that cannot be satisfied:
 ros-melodic-desktop-full: Dependent on: ros-melodic-desktop but it will not be installed
                            Depending on: ros-melodic-perception but it will not be installed
                            Depending on: ros-melodic-simulators but it will not be installed
                            Depending on: ros-melodic-urdf-sim-tutorial but it will not be installed
E: Could not correct the problems, you have withheld broken packages.

Does anyone know how I can get past this problem? Thank you in advance.

EDIT:

I have now succesfully installed ROS!!

I used aptitude for ubuntu - Advanced Packaging Tool (APT) system.

Does this mess up something considering path folders? By doing this I have no path called "opt/ros/melodic"... Now I cannot perform any environment setup. Meaning I cannot run the following instructions from ROS:

echo "source /opt/ros/melodic/setup.bash" >> ~/.bashrc source ~/.bashrc

bash: /opt/ros/melodic/setup.bash: No such file or directory

Please help me with this