1 Timothy 6:14-16 (NRSVUE)
14 to keep the commandment without spot or blame until the manifestation of our Lord Jesus Christ,
15 which he will bring about at the right time - he who is the blessed and only Sovereign, the King of kings and Lord of lords.
16 It is he alone who has immortality and dwells in unapproachable light, whom no one has ever seen or can see; to him be honor and eternal dominion. Amen.
Jesus is described as the one "who alone has immortality". Jesus is the antecedent in verse 14, and is called "King of kings" and "Lord of lords" in verse 15, just as in Revelation 17:14, 19:16.
Many Trinitarians would readily acknowledge that the Father is the subject of verses 15-16. They describe the one "whom no one has ever seen or can see". Many people saw Jesus, and Jesus is not described as one who people cannot see, yet God consistently is, often in distinction from Jesus (Exodus 33:20, John 1:18, 5:37, Colossians 1:15, Hebrews 11:27: 1 John 4:12). In chapter 1 of this epistle, it is God, not Jesus, who is described as "the King eternal, immortal, invisible" (1 Timothy 1:17), and the passage in question from chapter 6 can be seen as a callback to this, as both passages declare the Kingship, invisibility, and immortality of God.
Additionally, in chapter 2 of this epistle, the "one God" is unambiguously distinguished from Jesus (1 Timothy 2:5). So, the passage in question is actually stating that God alone has immortality (by nature), and is used as a Unitarian proof text.