I can apply paint to a cube, but not to my complicated object, even though I defined the material and I linked the image texture to the principal in the shader. It seems I cannot select anything in the UV map, so that might be the problem. (apologies but for some reason I cannot comment on my own question...)

[![enter image description here](https://i.stack.imgur.com/1l3fx.jpg)](https://i.stack.imgur.com/1l3fx.jpg)