Ever watch Critical Role, as the DM Matt Mercier has said on more than one occasion "you can always make a roll".
The problem with telling you when a roll is straight up impossible can break the game on occasion. Imagine a situation where the party encounters what they think is a mook, in reality it is a very high level NPC there to help them. He is one of those Elminster types that likes to guide others instead of act. He also has high enough stats that no one in the party has any chance of using Persuasion or Intimidation on him. Would break the game if they try to bribe him using Persuasion and I tell them sorry what you are doing is impossible don't even roll.
Basically in social situations in DnD you do not always know you are trying to do the impossible. It is the nature of social skills. It is obvious you cannot make an Athletics check to jump straight up 50 feet. It is not as obvious that you cannot bribe the head of the Kings personal guard who just happens to be his bastard son, a fact that will come out late. So when you try to bribe him you fail, it would would kill the situation if I told you not to bother rolling because you cannot succeed.
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