There is not so much info about how to add sponge plugins to forge.
All a know is that old version of sponge could load forge mod but newer version of sponge is instead sponge forge but where do you add the plugins and all the help and tutorial only show how to install and not add plugins.
You put plugins in the mods folder
Well I already tried this and forge said that plugin_name.jar is not valid mod jar and closed
Can you post the server log?
Is SpongeForge loading correctly? As stated above a server log is needed for further investigation
Found the error. The plugin was not working. but thanks any way