Unable to locate package ros-indigo-roscpp

edit

I am on ubuntu 16.04 [..]

and:

however with this command:

rosdep install --from-paths src --ignore-src --rosdistro indigo -y --os=ubuntu:trusty

I get this error

E: Unable to locate package ros-indigo-roscpp
ERROR: the following rosdeps failed to install
  apt: command [sudo -H apt-get install -y ros-indigo-roscpp] failed

Indigo is not supported / released on Ubuntu Xenial (16.04), which means that ros-indigo-roscpp doesn't exist there. Forcing rosdep to use the ubuntu:trusty defintions isn't going to change that.

You could try and see whether

rosdep install --from-paths src --ignore-src

after having sourced the correct (Kinetic) setup.bash works.

PS: this is exactly why I never really use --rosdistro $DISTRO in command line examples: the instructions will get copy/pasted by someone in the future on a different OS/ROS release and no longer work. --rosdistro $DISTRO is only needed if you don't have a setup.bash sourced anyway.

Edit:

I'm not sure what you mean by sourcing the correct setup.bash..

That would be source /opt/ros/$DISTRO/setup.bash. That will set the relevant environment variables, making --rosdistro .. unnecessary.

I decided to try to follow the instructions here: http://wiki.ros.org/tum_simulator#Ins... but to no avail, probably because I also have a different os version

Yes, those instructions are even older than the ones you found on dougvk/tum_simulator. Especially rosmake and friends are deprecated.

Is there a newer version of tum_simulator?

Not that I know of, but I'm really not into UAVs / quadrotors. You should probably spend some time searching for some, and / or try to contact a community. See Any quadrotor simulation package in ROS Kinetic? for a similar question.

But to get back to your original problem: can you show us the commands you've used and what their output was?