If you can not know that a statement can be true or false, you can also not believe it to be true. Moral relativism is a completely different subject, although I think that your position here is enlightening and provides a reason why you aren't understanding the rest of my position. The fact that we understand moral statements implies that they are therefore not nonsense and therefore either true or false. The only coherent attitude to take toward a predicate which you refuse to think has an extension is merely to assert that all statements which apply that predicate to any object are simply false. But this is not a problem simply for moral relativism, but for every relativism.
Truth is bivalent.
Every statement is either true or false. Anything which is not either true or false is simply not a statement.