It’s most likely that you aren’t setting up the necessary things. For example a Diffuse pass requires lighting. If there is no light, the texture will be completely dark.
Texture baking that involves lighting is slightly more complicated and often it’s not even what you wanted to do. Many people will bake ‘diffuse’ to get the surface colour, thinking that is what it is. But it’s not.
When you bake your diffuse in Blender, it will automatically apply lighting from the any ‘world’ texture you have, materials with an ‘emission’ component as well as any lights. You can think of the diffuse texture then as the mixture of all your lighting on the surface where it would produce a ‘diffuse bounce’. This isn’t the surface colour of the object at all. It also doesn’t factor in any ‘glossy bounces’, in fact if your surface is very shiny the diffuse output will also be completely black as it didn’t produce any ‘diffuse bounces’.
Bake Wrangler is interested in producing only the texture information you actually want. So, unless you specify things that should contribute lighting to the bake (using the ‘Scene’ input) it isn’t going to include any lighting. Which means you will always get a completely black texture if you have nothing in your ‘Scene’ input and try to do a pass that captures lighting information.
The actual surface colour of the object is ‘Albedo’ (again many people confuse albedo and diffuse as being the same thing) and contains no lighting information.
Any way, if you want to just do a test bake either include lights via ‘Scene’ input or use a pass that doesn’t bake lighting data (all of the ones listed as PBR).
Making the lighting stuff more intuitive, improving the documentation and probably making some video tutorials are all things somewhat on my radar.
