They’re able to say it. It’s an insult.
They can’t speak English though. In fact the scene this screenshot is from is them trapped in the past unable to communicate because their universal translators are offline. I would assume Quark speaks Federation Standard, but apparently that’s not close enough to English to figure out what they’re saying in the 40s
I would assume Quark speaks Federation Standard
I assume he made a clear point of never learning it.
I’d put larger odds on Nog and Rom eventually learning it (but not at the time of the screenshot).
My headcanon is that most of their speech is ferengi being translated by the universal translator, but when they say the word human they just use the human word for human which then goes untranslated.
It’s not even head cannon, this screenshot is legit not just from the episode, but from the scene where there’s an example of exactly what you described.