ANSWER: Even most of the old amil and covenant guys who know Greek know better. The passage should read, as is translated in the NAS, "the kingdom of God is in your midst." The kingdom was there because the King had arrived! But it is important to look first at verse 20 to get the full scope of what Christ is talking about. He said, "The kingdom of God is not coming with signs to be observed." "Signs" (semeion) had to do with some kind of "signal" in order to give a warning or fanfare of something coming. In my Luke commentary I write about these two verses:
"Signs to be observed" (Luke 17:20) is the verb parateresis, meaning "in a way that can be observed [be external signs]," with "critical observation," Jesus described His coming: "Just as the lightning when it flashes out of one part of the sky, shines to the other part of the sky, so will the Son of Man be in His day" (v. 24). No distinct precursor will take place when the kingdom comes. It will suddenly arrive on the scene!
"For behold the kingdom of God is in your midst" (v. 21). In every reference to the arrival, or the coming of the kingdom in the Gospels, the verb is in the perfect tense. In Luke the NASB translates 10:9, 11 correctly: "The kingdom of God has come near to you." It has arrived but it has not been inaugurated. "It has been on its way, and it has arrived, but it has in no way begun!"
In 17:21 Jesus continues this thought and told the Pharisees, "It [the kingdom] is existing in your midst, but it certainly has not started!" When kingdom of God (and kingdom of heaven) is mentioned, it is accompanied by a perfect tense, meaning the kingdom has come up to the moment but it has not been inaugurated. The kingdom itself will not come with signs (Greek, semeion) to be observed.
This lends itself to the fact that the kingdom, the millennial reign of the Messiah, has been postponed and temporarily set aside, but not permanently canceled or done away with. It will come at the end of the terrible tribulation period!
Thanks for asking.
Dr. Mal Couch