Own Minecraft plugin is not found?

I'm not a newbie when it comes to programming, because I've already written some programs (but in a different development environment). I recently switched to IntelliJ and I also find it easier. The problem is: I wanted to write a small plugin for my Minecraft server. That worked without any problems, only when I drag it to the plugins folder and restart the server nothing happens. Even with / plugins my plugin is not displayed with.

In the internet it was often said that it had something to do with exporting the plugin.yml, but I looked at different tutorials several times and did it exactly that way and it still doesn't work.

Could there be other causes?

(This would be my code. Just something very simple so I can see if it works.)

Which version is the server on which the plugin should run?
For IntelliJ IDEA there's a Minecraft class, plugin.

https://plugins.jetbrains.com/plugin/8327-minecraft-development/versions
or
https://minecraftdev.org

You can find this under Preferences >> Plugins.

I recommend this How2 for 1.16 plugins.

At minute 2:16 he downloads the Minecraft plugin for IntelliJ

I also advise against Bukkit or Spigot. There are other server services such as PaperMC. In my opinion, PaperMC is the best right now.

Thanks I made it now.

You helped me a lot!