Wulfara: I’m seeing the same thing w/Unity-2017.3.0f1 on Debian Linux 8.9 (Jessie). Depending on the Linux distribution you are using, support for monodevelop may already be available. For example, Debian Linux 8.9 (Jessie) supports the monodevelop package version 4.0.12 released in 2013, which is older than the latest stable release 6.1 SR1 (6.1.2.44) and can be installed w/dependencies by executing the following as root:
apt-get install monodevelop
After installation, launch Unity and set your preferences as follows:
Edit | Preferences | External Tools | External Script Editor: Browse…
navigate to /usr/bin/monodevelop
select [×] to close Unity Preferences
…then double-left-click any of your project’s C# scripts and the MonoDevelop IDE will launch. In the Solution navigator, expand the project tree and double-left-click on the .cs file to bring it up in the editor.
An alternative is to use Emacs IDE, setting your preferences as follows:
Edit | Preferences | External Tools | External Script Editor: Browse…
navigate to /usr/bin/emacs
select [×] to close Unity Preferences