You have run Spine without arguments, so it tried to start the UI. Try specifying command line arguments to perform a JSON or binary export, or to do texture packing. Any function other than those will not work on a headless machine.
http://esotericsoftware.com/spine-export#Command-line
Eg, this does JSON, binary, and texture packing:
./Spine.sh -e myJsonExport.json -e myBinaryExport.json -p myPack.json
The JSON files are saved from Spine's export dialog and texture packing settings dialog. Of course you don't have to do multiple exports in one run:
./Spine.sh -e myJsonExport.json
