How are you trying to apply a texture? What steps have you taken? To be fair, I have zero idea of your level of experience. At the most basic, you should 1) select the model in the scene, 2) open the Surfaces tab, 3) navigate to the specific surface group in the list, 4) select it, 5) navigate to the base color, 6) select the 'down arrow' icon which will offer an option to browse your database, 7) navigate to whatever image you wish to apply as a texture.
I assume you know all of this already, but one can never tell.