I expect to be able to "texture paint" in the bump-map slot on 2.8 and have my changes visually reflected -- i.e. if I paint a bump on the cube in the right-hand view, it should look like a bump immediately without changing modes. That expectation comes from this online tutorial, although it seems to be using an earlier version of 2.8 with different interface widgets.
But that's not what happens: when I paint a bump in "Single Image" mode: ... then I always get the error message shown in the top picture: "Missing Textures, detected!" However, if I switch the mode to 'Material': ... then the brush color is reflected on the cube, but it's shown as a color change, not a change to the normal height. I can switch modes to see the change as a bump, but that's tedious and makes it difficult to paint any complexity for all the mode-switching.
I've ensured that there are image files for the diffuse and bump map, and they've been saved before the experiment is attempted.
This problem means that it really isn't possible to interactively 'paint' bumps in a visual way, because I cannot see the effects of my brush without changing modes. It seems clear that this is not the design intent of the software.
Am I doing something wrong, or am I seeing a bug of an unfinished product?