“…behold, the kingdom of God is in the midst of you.” Luke17:21 English Standard Version [also RSV, NASB, NIV, CSB]
The kingdom of God is God’s reign; his rule. It is both present and future; inaugurated with the ministry of Christ but not yet consummated. As in the title of George Eldon Ladd’s book, it presently is The Presence of the Future.
In Christ, the kingdom has come near (Mat, 12:28; Luke 21:31, etc.). Some enter it; some do not (Mark 10:23ff.). See the ISBE, s.v. Kingdom of God
As the forerunner before Christ’s ministry, John the Baptist cried, “Repent, for the kingdom of heaven is at hand!”
Jesus began his ministry with those same words, following the temptation in the wilderness (Mat. 4:17).
In Luke, in response “to the Pharisee’s question, ‘When is the kingdom of God coming?’, Jesus can therefore answer, ‘The kingdom of God is in the midst of you” (Lk. 17:20f.; not as AV ‘is within you’).”—The New International Dictionary of New Testament Theology, s.v. King.
[Jesus is not telling the Pharisees that the Kingdom of God is ‘within’ them. He warns them, “The tax collectors and the harlots go into the Kingdom of God before you” (Mat. 21:31). And he rebukes them, “You shut up the kingdom of heaven against men; for you neither enter yourselves, nor allow those who would enter to go in“ (Mat. 23:13).]