I'm using Blender 2.9alpha, Cycles. I have a 2d plane with UVs. I created a nice procedural marble texture, feeding Base Color in Principled BSDF. Looks great. Then I try to bake it to an image texture, but I get all black. I created the texture image, added the image-texture node to the material's node window (unconnected but selected), selected Combined (tried others too), and the render takes a while, no errors, but all black. If I bake out UV I get a nice UV color wash, that's good. But to bake the actual texture, nothing works. Diffuse should work if I just want the procedural texture that I've applied to the Base Color, right? (I'd paste a pic of my shader nodes but imgur seems to be down.)
The normals of the plane object are facing the wrong way (downwards). To correct this: in Edit mode select the plane then open the Mesh menu > Normals > Flip .
Note: because your image to bake to (dark marble texture) was not packed into the .blend file I had to create a new one that is called new_bake in the screenshot below.