The problem, as the logs say, is the texture format is unsupported. The solution is:
For every texture you will want to export, go to the texture asset (from the content browser), go to “Compression”>“Compression Settings” and set it to UserInterface2D. This will set the texture format to RGBA, which is compatible with the PNG or JPG exports. (You could also try some other settings if you want). This will unfortunately make the texture bigger in size, resulting in a heavier packaged size.
Hope this helps