"Amateur" probably sounded weird to you because you might have wrote it as "Amateur people." If that's the case, that's the reason. People would just say "Amateurs." Just restructuring the English of that box, it should sound more natural to say "Amateurs shouldn't be handling a gun."
If you're wondering, the "Nipple game" is where you poke at someone (who has obviously given you consent to do this) over their clothes and try to guess where their nipples are at.
That's it. That's the joke.
I've memed this so many times, but I have never ACTUALLY hated something so intensely that I can't stop reading. I thought it was a made up feeling for the longest time. But I HATE this manga, and I genuinely can't stop reading it.
I wonder if you could run pirate software on the Mina to disable the ad. That definitely seems like a very lazy think that could be circumvented by most people, as I imagine the devs would likely be too focused on making sure the robo-cleaner doesn't go haywire.