How to import single player minecraft world into server folder

The world folder has to be called "world".

In general, if you are in such a situation, you can start with the default world, then copy the parts of the world one by one and see if they work. That way you can narrow down the cause.

Place the world folder from the saves directory into the root directory of the server. (the same folder the server file is in)

You have to change level-name in the server.properties file to match your world.

Bukkit will split the world into three folders which contain the overworld, the nether, and the end.

If you have done that, I would suspect multiverse has to be configured.