The difference between the two sentences, “Is it possible you…. ?” and “Is it possible if you….?” is very small – only the word “if”. In certain circumstances, they can both be correct! It depends what you want to say. As long as you use it in the correct circumstances, both can be used.
For the first option, you are wondering whether it is possible that you did something. For example, maybe you distinctly remember making a reservation for a car rental. However, when you get to the car rental place, they are unable to find your reservation. If you are discussing reasons that you think you have made a reservation but the car rental company cannot find it, they can ask, “Is it possible you made the reservation with another company?” or “Is it possible you canceled your reservation or made it for the wrong day?”
The main idea is that you are questioning the person, “you”. You are trying to determine whether this “you” person has done something or not.
For the second option, the emphasis is on “it”. This means that you somehow are referencing something that you have mentioned before. For example, you might be trying to spend Christmas with your family, but your boss is not letting you have enough vacation days. You could ask “Is it possible if you take days off from your annual leave?” or “Is it possible if you just spend one or two days with your family?”
The main idea here is that you are trying to come up with solutions to a problem that you have already mentioned before.
In short, both sentence structures are grammatically correct. The one that you should use depends on what you are trying to say!