Hello.
I am following a book, building a simple model with high/low poly.
The high poly model’s UV is OK, but the low one I do same step is not work.
After step Mark Seam, Select All, Unwrap’s result on UV editor is just a point.
Where is error?
You’ve pinned all the vertices to the same spot. You probably accidentally hit P in the UV editor. Simply move your mouse to the UV window and hit alt-p to unpin them, then re-unwrap.
If you look in the UV editor and see red, that’s a pinned vertex. Or many pinned vertices, in this case. If you didn’t intend to pin anything and you see red, that’s the problem.