By default blender allows you to scale past 0 - how do I prevent this?
i.e. it's common in other 3D modelling software to resize a set of vertices against an axis to align them - however this isn't possible when negative scaling is enabled.
As far as I know this isn't possible, use the keyboard input instead.
After pressing S for scaling use the keyboard to enter 0 (zero) to perfectly align geometry and optionaly choose an axis of alignment by pressing either X Y or Z.
Select the mesh in OBJECT mode - Object Tab - Apply cntrl A - Rotation & Scale.
The error is coming up because when you're unwrapping it, it doesn't like the rotation and scale.
IDK how I just figured that out myself considering I was looking up the same issue and remembered scale can mean the rotation/scale/orgin of a mesh as well.