I have my own Minecraft server and wanted to enable players to create shops via plug-in, but only OP players can create a shop if a non-OP player tries to create one, only the message comes that he does not have the authorization to do so.
Where do I have to change that? Somewhere with the plugin itself, with vault or somewhere else?
You need a permission plugin for this, e.g. https://www.spigotmc.org/...rms.28140/.
The required permissions are available on the plugin page.
Then create a group and give permission to create a shop.
