Mods and Plugins
General Minecraft Software Rules:
Vanilla Minecraft can load Minecraft (Not mods and not plugins).
Bukkit-based Minecraft can load plugins (Not mods).
Forge-based Minecraft can load mods (Not plugins).
Hybrid software can load mods and plugins, however, depending on which software you opt for is unsupported, depreciated and unstable.
Note: 1.10.x+ mod servers can use SpongeForge to load plugins.
Unable to open file stream
The plugins and mods directory is created when the server is first run with the appropriate folder.
If you get the error "Unable to open file stream" (or similar) when attempting to install mods/plugins via the mods/plugins menu, this means the panel cannot find the folder to install to; Make sure your server has the appropriate directories setup by starting the software via the console first.
Note: You can use the Files menu, or sFTP/SSH to create a mods/plugins folder manually, however, if you do not have the correct software (mentioned at the top of this page) installed, you will not be able to load those mods or plugins.