I can't create any commands.

[LO1ZB]

Player "myplayername" tried to execute command "/j" that was stopped by the HOOK_EXECUTE_COMMAND hook

function Initialize(Plugin)
        Plugin:SetName("STONED")
        Plugin:SetVersion(1)
        
        -- Hooks
        
        cPluginManager:AddHook(cPluginManager.HOOK_EXECUTE_COMMAND, RollJoint);
        
        PLUGIN = Plugin
         
        -- Command Bindings

        cPluginManager.BindCommand("/j", "STONED.j", RollJoint, " - is rolling a joint."); -- RollJoint binding       
        
        LOG("Initialised " ..Plugin:GetName() .. " v." .. Plugin:GetVersion())
        return true
end

function OnDisable()
        LOG(PLUGIN:GetName() .. " is shutting down.")
end

-- Default callback name is: function OnExecuteCommand(Player, Command)

-- RollJoint Command: 
function RollJoint(Player, j)
        SendMessage(Player, " " .. Player:GetName(Player) .. " is rolling a joint.")
        return true
end

Why do you use the same function for a Hook and a command?
It is called first from the hook, then you after sending the message to the player, you are returning "true", which means "hook successfully executed", if you want that the command get executed, you have to return "false".