I think most people would say not letting you attempt to do something because they think your character can't possibly have enough of a bonus to do it is railroading. Again, like I said, I don't have foresight to know what the bonus might be. What if the bard decides to inspire them? What if the cleric uses guidance before? What if they have some item that gives them a bonus and they haven't written that in and just add it in the fly?