Second, regarding Discover. I assume Discover is what you are attempting to use here. OM is different from other distros. Discover is not used as a package manager. Therefore, the error and crash. From the terminal, I use dnf. If you desire a GUI package manager, use Dnfdrake instead of Discover. I have used Dnfdrake and it works as advertised. About the only thing that I use Discover for is to update the Plasma theme components currently in use. Most of the time, Discover makes for a great virtual doorstop.
Third, Synaptic is the GUI package manager for apt. OM uses dnf. That would explain why you are having trouble. OM uses Dnfdrake.
Fourth, why Nemo? It is the file manager for the Cinnamon desktop environment. Plasma uses Dolphin as the file manager, and it should be installed when the Plasma desktop environment was installed. Perhaps someone else can help you with installing the Cinnamon desktop environment. Is it possible to install? I don’t know. I tried to use it years ago and got frustrated with it in under an hour. Ever since, I have been using Plasma as my desktop environment. One feature of Plasma is I can make it look and feel exactly the way I want. Right now I have one desktop on one laptop looking like Windows 2000. The other laptop’s desktop (this one) is a minor train derailment cleanup in progress.
Always fill out the template correctly, and include if you did what we asked (i.e. searching for things like Discover not working or supported and how you should be installing, updating, and managing repos). Most of what you are asking is already on the forum.
The cinnamon packages don’t have a full time maintainer. So you may need to go back to Linux Mint if cinnamon is a requirement of yours. We will also never package synaptic. It doesn’t work with dnf/rpm at all in this distro. You may be able to user an older version of PCLinuxOS, but they are also getting away from synaptic/apt.