I did a big long post about what this is and why it's happening but the forum decided to kick me out and now I've lost it so now you just get the TL;DR
Go into "Manage extensions" -> enable "Developer mode" in the top right hand corner.
This setting does not take immediate effect. You may have to refresh the page a few times or quit and restart Chrome.
You can see if it has worked by opening the Tampermonkey menu and making sure the message has disappeared.