11-The Pharisees came and began to question Jesus. To test him, they asked him for a sign from heaven. 12-He sighed deeply and said, "Why does this generation ask for a miraculous sign? I tell you the truth, no sign will be given to it." 13-Then he left them, got back into the boat and crossed to the other side.
38-Then some of the Pharisees and teachers of the law said to him, "Teacher, we want to see a miraculous sign from you." 39-He answered, "A wicked and adulterous generation asks for a miraculous sign! But none will be given it except the sign of the prophet Jonah.
Christians might try to say that Jesus meant that they would not receive a sign from heaven in Mark 8, however Jesus made it clear that they would not receive any sign and did not specify which kind. Jesus said no sign will be given to them. However, we don't see this in Matthew.
This is the Biblical version of "show me
your papers". The Pharisees wanted objective
proof that Jesus was who He said He was.
Basically, what they wanted was a magic trick. Of
course, if they had been with Him for any length
of time, they would have already seen healings
and other miracles. But they wanted miracles on
demand like Pizza Hut. If you had continued to
the next verse of Matthew, the meaning of this
passage would be clear.
Matthew 12:39 But he answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas: Matthew 12:40 For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth.
In other words, Jesus was telling them they would get their sign when He rose from the grave. But to give them a miracle on demand would be perilously close to the experience of Jesus when He was tempted in the wilderness. You recall that Satan told Him to prove Himself to the people by diving off the cliff so that the angels would catch Him and save Him. Jesus replied: "It is said, Thou shalt not tempt the Lord thy God" (Luke 4:12).
Jesus was not going to let Satan tempt Him and He wasn't going to let the religious professionals get away with it either.
Hope this helps.