Does anyone know which plugin send this message, please? NOTE: When I'm an Operator (OP) the message does not appear
Essentials(Chat?) has a configuration field for welcome messages, so I would check its config.yml to see if it's set to what you're seeing; I believe there's an option to disable it.
Just to be sure, did you restarted server right? So it is quite difficult to guess which plugin it might be. I recommend to try to search all plugin configuration files (for example via Notepad++ which allows to search in multiple files) and look for key section like "has joined the Server!"
I`ve fixed the problem the plugin was DeathMessages! Thank you all for trying to help me I appreciate it