To be honest, it’s the requirements of installing Forge…you have to download the Vanilla Minecraft Server jar as well as Forge, then open the Vanilla jar, delete a folder, then drag the files from the Forge jar over into the Vanilla jar. Forge’s auto-installer for servers still only downloads Forge and the Vanilla jar and leaves the rest of the work to you (modifying the vanilla jar with forge files). The reason Spigot/PaperSpigot works is because it actually builds a server-ready/complete Minecraft Server jar, which can be run right after it’s process if done.
Easiest Solution: Download the Forge auto-installer on another computer, select server install and choose a directory; when complete, open the Minecraft Server jar (open, not run) and delete the “META-INF” folder, then open the Forge jar and drag-n-drop the contents into the Vanilla jar. When complete, you now have a Forge Minecraft Server jar, which you can now just drag-n-drop into your server’s base directory (if MineOS doesn’t show the jar, click on your username and refresh server list then try again)