St. Gregory says
By the kingdom of God is meant the present Church, and because some of His disciples were to live so long in the body as to behold the Church of God built up and raised against the glory of this world, this comfortable promise is given them, there be some of them standing here.
The Blessed Theophylact had a different view which is also very possible
He had said that the Son of Man would come in His glory. So that they would not disbelieve Him, He says “there be some here” who would see, as far as they were able, the glory of the second coming in the Transfiguration. At the same time He shows what great glory will belong to those who suffer for His sake. For as His flesh shone like lightning on that occasion, so in due proportion will the saints shine forth then at His second coming. Here He is hinting at Peter, James and John, whom He took with Him on the mountain and showed them His kingdom, that is, the future condition in which He would come and both He and the righteous would be radiant. He is saying, therefore, “Some of you here shall not die until you have seen Me transfigured.” See, then, that it is those who stand firm in goodness who see Jesus radiantly transfigured, and they are ever advancing in faith and in the commandments.
I lean more towards what St. Gregory said.