After I installed 18.10 (when it was beta) and started to install all software I got this problem with MyPaint. Today this problem still persists. Universe repository is enabled. I was able to install it from flatpak thought.
sudo apt -f install doesn't see any problems.
When I'm trying to install MyPaint:
$ sudo apt install mypaint
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:
mypaint : Depends: mypaint-data (>= 1.2.0-4.1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
$ apt-cache policy mypaint mypaint-data
mypaint:
Installed: (none)
Candidate: 1.2.0-4.1
Version table:
1.2.0-4.1 500
500 http://archive.ubuntu.com/ubuntu cosmic/universe amd64 Packages
mypaint-data:
Installed: (none)
Candidate: 1.2.0-4.1
Version table:
1.2.0-4.1 500
500 http://archive.ubuntu.com/ubuntu cosmic/universe amd64 Packages
500 http://archive.ubuntu.com/ubuntu cosmic/universe i386 Packages
This command line resolves the problem but it removes Gimp (and installing Gimp after MyPaint will remove MyPaint).
I have the same problem on Ubuntu 18.04 and before on Fedora 26 and I didn't find any solution except swipe to another simple painting program like "Kolourpaint" you can install it from Ubuntu software (Gnome software center) or by executing the following command: