about building height problem

server.properties file :

max-build-height = 256

Change that value.

 

You can't build higher than 256 blocks, it's a minecraft limitation.

 

As Norska said you can't go higher than 256.
The issue is the client won't recognize it nor would it render the block. So even if Spigot were to allow the build to go over it (which technically it could) the client wouldn't show any of the blocks.

That number is only for lowering. If you want to increase, you would need to use a forge server and have mods to change the value on both server and client.