Is it allowed to call upon Allah by saying “ya Rab” because I think I saw a fatwa from sheikh Rab’ee saying you can’t.

Yes it is allowed, and it is the form of duʿāʾ Allah Himself teaches in the Qurʾān (rabbanā, rabbi). so bring me the fatwā if you find it, as you might have misunderstood the sheikh may Allah have mercy upon him.