Why can't I unwrap this model?

Hi, I’m just getting started with Blender and I made my first model.

However I’m not sure what I did wrong, but it seems to be unwrappable…??

pb_wrench.blend (471 KB)

Any help would be appreciated!

What do you mean by unwrappable. ?

When you open a UV/Image editor window you may first have to remove the ‘Render Result’ if that is what is currently show by hitting the X button. Then you would be able to see the unwrapped mesh.

To unwrap you should add seams (Ctrl+E / mark seam) to aid unwrapping or use U / Smart UV Project. You should first clean up your model by removing any of the ngons in your model so you just have quads/tris as this will be less prone to unwrap errors

In 3D View Edit mode press A to select all faces, then U and Unwrap, then you should see it in UV/View Editor (if faces are still selected), but as Richard said Render Result need to be closed. Also for proper unwrapping you should mark seams before pressing Unwrap.

So where do I mark seams on a model? Assume I have a character in t-pose. Do I mark seams along the side or front? Is seam marking done in edge selection mode?

So where do I mark seams on a model?
Does your model have a natural seam (garments have seams so why not use the knowledge tailors have built up over the years by using the same seams in your garment models). Put them where it would give minimal stretching of the UVs and ideally as hidden from view as possible, most figures are usually viewed from the front, so move seams to the back of the character, around the crotch, inside leg, under arms etc. Either as sperate islands or one whole piece such as:

Is seam marking done in edge selection mode?
Edge, vertex or face select, most usually edge selection