Is a man sinful if he does not accept the statement of one who informs him that doing something is forbidden or obligatory by Sharia, due to the lack of evidence mentioned for that?

A Mufti must be knowledgeable in what he gives fatwa about, because issuing a fatwa without knowledge is a fabrication against Allah and a deviation. If the speaker is not among the people of knowledge, then his statement should not be followed. However, if he is a well-known scholar, then if the questioner is a student of knowledge, it is incumbent upon him to seek proof, and if he is an ordinary person, it is incumbent upon him to follow the Mufti's opinion. The Mufti should mention the evidence for the ruling and its source whenever possible.