Loading a schematic and pasting it are separate steps. First check that the file is in the schematic directory used by the server plugin, rather than a folder on your own client.
Check discovery, format and placement
In the example, arena is the schematic name. If it is absent from the list, check the directory, filename and permissions. If it is listed but fails to load, look for format or block-data errors in the log.
//schem list
//schem load arena
//paste -n
On supported WorldEdit versions, //paste -n selects the paste area without changing blocks. Placement depends on the relative origin stored when the clipboard was created. Check the selection before committing a paste.
Use a test world for the first actual paste. Renaming an old .schematic file does not convert its format.