sorry, should be listed as just "mods" on the software page, or "forge"
Thank you. When I access the Forge page for Minecraft version 1.15.2 (which is the version I am using), it indicates to me that it is already installed since it shows the "Reinstall" button instead of "Install:"

Perhaps I cannot access the Quark mod page because the site is not detecting that I have the correct Forge version installed on the server. I will next back up my current world and plugins, then reinstall Forge for Minecraft 1.15.2 to see if this resolves the issue of "file not found" when I visit the Quark mod page that you linked above.
Update 1: I searched the Modpacks section of the Software page, in case a modpack contained the Quark mod. After searching first 'Curse,' then 'Others,' and then the rest, I did not find the Quark mod in any of them.
Update 2: I have successfully created a backup of my plugins and world, and have re-installed Forge for Minecraft version 1.15.2.
After this, I went to the Quark mod page which you linked above, and I now see the Quark mod! I have successfully installed it along with its dependencies, and am running the server. It appears that the issue was either my Forge installation was not being detected previously, or my installation of Paper/Spigot was interfering somehow.
Thank you for sticking with me as we shot the trouble of this issue, Bluewy!