The following versions have the sign in heaven:
Matthew 2430 (New King James Version) Then the sign of the Son of Man will appear in heaven
Matthew 2430 (New American Standard Bible) And then the sign of the Son of Man will appear in the sky...
Matthew 2430 (English Standard Version) Then will appear in heaven the sign of the Son of Man
The following versions have the Son of Man in heaven:
Matthew 2430 (Revised Standard Version) then will appear the sign of the Son of man in heaven
Matthew 2430 (1901 American Standard Version) and then shall appear the sign of the Son of man in heaven
Matthew 2430 (New International Version) At that time the sign of the Son of Man will appear in the sky...
Matthew 2430 (Young's Literal Translation) and then shall appear the sign of the Son of Man in the heaven;
Matthew 2430 (Concordant Literal Translation) And then shall appear the sign of the Son of Mankind in heaven, and then all the tribes of the land shall grieve, and they shall see the Son of Mankind coming on the clouds of heaven with power and much glory.
Matthew 2430 (King James Version) And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory.
In bold above what does YLT, CLT and KJV say the sign is? It is the sign "of the Son of man." It is not, the sign in heaven. It is the sign of the Son of man in heaven. Not, the sign in heaven of the Son of man. There are variants in the translations consulted, so I have to grant the possibility it could be the sign in heaven; yet, the most literal versions for which I have more respect do have the Son in heaven. This is what I've discovered so far. Until now I'd never consulted multiple versions on this phrase. If the sign is in heaven we lose the connection with Daniel. Then too, what is this sign? I understand that the sign that Jesus was the real thing is His reigning from heaven.
Jesus' words are in reference to Daniel 713-14. Jesus' ascension is in view, both in Daniel and Matthew. The fact that His prophecies in Matthew 24 were fulfilled proved He was the Son of man to which Daniel referred. Nobody could destroy God's Israel that He had created, disciplined and protected. The one that had prophesied it said they would see Him, the Son of man present in clouds and glory. His presence was before the throne of God, receiving dominion, glory, and a kingdom. The people of Israel found out He was right and He was indeed the Messiah, the Son of man and the Son of God when His words were fulfilled.
Daniel 713-14 King James Version  I saw in the night visions, and, behold, [one] like the Son of man came with the clouds of heaven, and came to the Ancient of days, and they brought him near before him.  And there was given him dominion, and glory, and a kingdom, that all people, nations, and languages, should serve him: his dominion [is] an everlasting dominion, which shall not pass away, and his kingdom [that] which shall not be destroyed.