My iPhone 7 video edit tools don't work

People keep talking about a timeline at the top of the screen with a yellow trim button used to trim videos. I don't get any of that. I have a timeline at the bottom of the screen. There is a slider, but after I move the slider to the correct trim position, there is no button allowing me to actually trim the video. My video is 26 seconds long and I want to trim it to 23 seconds. The only option I have after dragging the slider is "Cancel." What am I missing?????