"There is a taxi outside the hotel."
You don't need 在 at the front in this sentence.
Using 在 at the front you could say something like
would it be correct to use 是 instead of 有? Some of the other apps teach that both are right. Is one more or less natural/better fit for certain circumstances?
是 is usually used to describe a property of an object or make a judgement about the property or character of an object, e.g. water is liquid=水是液体 question---'is it a taxi or a bus outside the hotel?'（酒店外是一辆出租车还是公交车？） answer---'it is a taxi outside the hotel'(酒店外是一辆出租车。)
有 is usually used to describe a condition of current situation or make a judgement about the existence of an object, e.g. question---'what's outside the hotel?'(酒店外有什么？) or 'is there a taxi outside the hotel?'（酒店外有一辆出租车么？） answer---'there is a taxi outside the hotel.'(酒店外有一辆出租车。)