Its a story as old as Android Oreo the update is announced, it starts rolling out and then someone hits pause. It has happened to several phones now, including LG V30 phones on T-Mobile US. But it is all okay now, V30 and V30+ phones can once again update to Android v8.0.
Before you head to Settings > About, check that youre running McAfee Security for T-Mobile version 4.9 or higher. Otherwise, the update will not be available to you.
Also, note that T-Mobile has set daily limits to limit network congestion. So if you cant download the update today, try again tomorrow, it could be because the limit was reached. Those limits will be lifted on July 3.