I can’t be sure, but it seems to me that the beginner tutorial for making a material, really ought to point out what texture files are allowed, and which ones are not.
Perhaps, the reason why my exported cube with a png file as diffuse texture looked like a “texture missing” cube in Lumberyard, is because the Lumberyard Editor only works with dds files or whatever it is called.
I think, as a new user to the Lumberyard Editor, it is to be expected that the editor has some kind of converter, that convers the file format used as texture, into whatever file format is actually required, and more, that this requirement is actually stated.
I knew already that the editor that the Lumberyard Editor was based off, required DDS files, or whatever they are called, but I did not expect that this requirement would simply follow over to the new editor.
Another problem, was that my imported object in the Lumberyard Editor was too small to be seen, because the default unit size of the exported object from maya (centimeter) is not the same as the default unit size of Lumberyard (meter). I figured out that instantly, but others would be confused by this. The tutorial ought to mention that importing objects exported from Maya, might require upscaling objects by a factor of x100 to become visible.