Matthew 12:39

But He replied and said to them, “An evil and adulterous generation (that is morally unfaithful to God) craves and demands a (miraculous) sign; but no sign will be given to it except the sign of the prophet Jonah;”

The demand for a sign means that unbelief is present.  A generation that is in sin doesn’t want to see the error of its ways or has faith in God.  Faith makes all the difference in our lives in what we see and believe.  We don’t need to see a sign for faith because we already have the answer by faith in our hearts.