The WTBTS says it is on earth. However, an examination of the 3 scriptures that mention paradise raises questions as to the validity of the WT's claims.
For instance, at Luke 23:43 Jesus tells the criminal beside Him that he (the criminal) will be with Him (Jesus) in paradise. The WTBTS asserts that Jesus can never return to earth so where is the paradise Jesus was referring to?
Luke 23:43 You will be with me in Paradise.”
Paul, at 2 Corinthians 12:3-4 says that unutterable words which it is not lawful for a man to speak were heard in paradise. So who could speak these words and where would they be spoken if man could not speak them?
2 Corinthians 12:3-4 Yes, I know such a man-whether in the body or apart from the body, I do not know, God knows- that he was caught away into paradise and heard unutterable words which it is not lawful for a man to speak.
Jesus said, at Revelation 2:7 that to him that conquers Jesus would grant him to eat of the tree of life in the paradise of God. In Revelation, the only reference to ones who conquer are those who have the assurance of heaven. So is the tree of life on earth or in heaven?
Revelation 2:7 To him that conquers I will grant to eat of the tree of life, which is in the paradise of God.’.