Plugins and mods are allowed to depend on each other. When one is missing, the console names it precisely — which makes this one of the easier errors to fix once you know where to look.
What you are seeing
Could not load 'plugins/Something.jar' … Unknown/missing dependency: [Vault]Missing or unsupported mandatory dependencies- One plugin fails while all the others load normally.
Why it happens
Many plugins build on shared libraries — Vault for economy, PlaceholderAPI for text substitution, ProtocolLib for packet handling. Mods do the same, often on API mods. Without the dependency present, the code that needs it cannot be loaded at all.
What to do
- Read the name in brackets
The log names the exact dependency. That is the thing you have to install, at a version compatible with your Minecraft version.
- Download it from the official source
The dependency's own page. This is the situation where people reach for the first search result and end up with a modified jar — there is an article on why that ends badly.
- Restart rather than reload
Dependencies are resolved at load time. Reloading a server after adding one frequently leaves it in a half-initialised state; a full restart does not.
- Check the versions match
A dependency built for 1.20 will not satisfy a plugin running on 1.21. The log will say so, sometimes in a second, different message.
The plugin appears in plugins output in green, and the missing-dependency line is gone from the startup log.
If the dependency is itself already installed, look at load order and versions: an old copy of the library sitting in the folder can satisfy the name but not the version the plugin asked for.