This happens every time I shoot a portal the level I created. What could the problem be? I don't got much more to explain this. I'm too noob!

I also would like to welcome myself to the forum! Can't wait to work on some of my own maps!

Yeah, I get that too. But I don't think it's something to worry about.

Its the error metal walls give to tell the engine that portals are not allowed on that surface. If you are getting it on white surfaces and portals DO show up then IDK... glitch?

As for why it shows up at all, maybe developer oversight or debug test? To turn it off type "Developer 0" in the console. As ml said there is nothing wrong with the map, everyone gets it.