I had a fresh new installation of Ubuntu 16.04 and first thing I did was install Unity3d 5.3.4f1 as my team develops on that version. But when I tried to debug in monodevelop, I came across these 31 errors and 1 warning which was awkward. This is the only thing keeping me from switching my project to Linux right now.
I tried checking References in the solution and it seems to show System, system.xml, system.xml.lang and those things added there already
Try installing MonoDevelop (sudo apt-get install monodevelop), it should install any packages it requires to function properly.
Update: I have fully fixed Unity3d and monodevelop to working condition in Ubuntu 16.04. Im attaching my finding. Good luck:
Unity3d Fix:
mkdir -p ~/.local/share/unity3d/Packages
sudo apt-get install -y libunwind8 libcurl3
wget http://security.ubuntu.com/ubuntu/pool/main/i/icu/libicu52_52.1-8ubuntu0.2_amd64.deb
$ sudo dpkg -i libicu52_52.1-8ubuntu0.2_amd64.deb
$ sudo sh -c ‘echo “deb [arch=amd64] https://apt-mo.trafficmanager.net/repos/dotnet/ trusty main” > /etc/apt/sources.list.d/dotnetdev.list’
$ sudo apt-key adv --keyserver apt-mo.trafficmanager.net --recv-keys 417A0893
$ sudo apt-get update
$ sudo apt‐key adv ‐‐keyserver hkp://keyserver.ubuntu.com:80 ‐‐recv‐keys 3FA7E0328081BFF6A14DA29AA6A19B38D3D831EF
$ echo “deb http://download.mono‐project.com/repo/debian alpha main” | sudo tee /etc/apt/sources.list.d/mono‐xamarin.list
$ sudo apt‐get update
$ sudo apt‐get install mono‐devel mono‐complete ca‐certificates‐mono mono‐xsp4
$ sudo apt‐get install monodevelop=5.10.0.871‐0xamarin2
