Spine's CLI parameters are configuration, then an action. You can have multiple actions:
[config for action] [action] [config for action] [action]
Your command line is:
-i images -o ./pack -n spineboy-ess -p ./pack.json -j ./spineboy-ess.spine
The first action is that you pack:
-i images -o ./pack -n spineboy-ess -p ./pack.json
Then you specify -j, which is short for --project and gives the packer context so it can pack meshes intelligently. However, there is no action after the configuration:
-j ./spineboy-ess.spine
If you want the project context for your packing, it must come before the pack action -p:
-i images -o ./pack -n spineboy-ess -j ./spineboy-ess.spine -p ./pack.json
