Yes, you totally are allowed to complain even if you don't vote. And no one's there to stop you.
I don't know why people keep saying this, because it's outright silly. Having the right to vote includes being able to choose not to vote at all. No where it is written that you forfeit your right to question and criticize the system if you refuse to vote.
You're allowed to complain that the only candidates that stand any conceivable chance of winning are both scum and lack the integrity, leadership, and honesty to be the USA's next presidential leader. You can also refuse to vote basing that choice on your disgust that you either have to vote for one of them and essentially be in part responsible for all of their actions, or essentially throw your vote away on a party that realistically will not win by any odds.
Not voting (which is what I'm doing) can be done on the basis that it is your full and complete rejection of the only viable candidates and to express full dissatisfaction of the system that is in place.
No matter who wins tomorrow, do you think that's going to stop me from complaining and questioning leadership if I see said elected official partaking in activity that I feel is wrong, unprofessional, and dishonest? It certainly won't.
What reason is there to believe that the candidate that has over 30 years of experience lacks leadership?