I'm currently on Ubuntu 18.10. I have followed all the instructions here and at https://godotengine.org/qa/28667/godot-mono-c%23-in-ubuntu-bionic?show=28667#q28667 , and am unable to install mono 5.12.0 on my machine.
deb https://download.mono-project.com/repo/ubuntu bionic/snapshots/5.12.0 main
When I attempt to install mono-devel, I get:
sudo apt install mono-devel
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
mono-devel : Depends: mono-runtime (>= 3.0~) but it is not going to be installed
Depends: libc6 (>= 2.27) but 2.26-0ubuntu2.1 is to be installed or
libc6.1 (>= 2.27) but it is not installable or
libc0.1 (>= 2.27) but it is not installable
Depends: libglib2.0-0 (>= 2.56.1) but 2.54.1-1ubuntu1 is to be installed
Depends: libmono-cecil-private-cil (>= 126.96.36.1991) but it is not going to be installed
Depends: libmono-cecil-private-cil (< 188.8.131.522) but it is not going to be installed
followed by a whole lot more equivalent stuff. I have tried turning off my other repositories, and still get the same result.
It looks like the problem is that everything mono is trying to do is depending on libc6 >= 2.27, and ubuntu has libc6 2.26, but I'm not sure if that's the root of the problem or not. I tried setting it to point at the older version of 5.12.0 (184.108.40.206), and had the same problem.