I watched this tutorial. https://www.youtube.com/watch?v=5NlvgMYZgaY In the video, a YouTuber creates an uv map that helps him position mouth. In the video, the shape of the mouth changes according to the position when the YouTuber moves using the uv warp, but my mouth shape doesn't fit the position. Why isn't the uv warp working properly for me?
Just apply location and scale of the armature with Ctrl+A, Location and Ctrl+A, Scale - or just use 1x All Transformations which applies also the Rotation.
Location should be (0, 0, 0) and Scale should be (1.0, 1.0, 1.0) in the N-panel's Item tab. The same data can be found in Object Properties > Transformations.
Do the same for the Scale of the mouth plane which is slightly scaled up (1.016).