'I cannot just wait for it' puts emphasis on the fact that you have to wait and do nothing, it implies that you'd want to do something else whilst waiting. The sentence would make more sense if it said 'can't' rather than 'cannot'. 'I just cannot wait for it' puts emphasis on the fact that you cannot wait, it expresses excitement for whatever is coming.

I agree with Bee Moore... the position of "just" in a sentence makes a difference. consider this also: "I was just doing my job." - meaning, I wasn't DOING anything but my job. "I was doing just my job." - meaning, I wasn't doing ANYONE'S job (minding my own business)... :)

"I cannot just wait"..."Just" here means "only", and refers to only waiting. She doesn't want to do only waiting, she wants to do something else as well. "I just cannot wait" implies not only "cannot", but also the desire that she doesn't want to wait.