Robin not knowing haki is entirely on her.
she was with Dragon ffs. And sabo and koala literally offered to teach her stuff too. But she didn;t wanna and only relented to learn a few fishman karate moves.
I think the reason she wasn't allowed is because with haki, she would be a formidable fighter. She could have even eclipsed snaji and zoro. Sanji is obvious. And zoro, she can literally use 100 sword style. As we know, more swords=more power. Robin woulda negged them all.
So basically, oda was afraid she would overshadow the m3.