Join two faces

Hello, I have to join this two triangles into a square. So I could fix the painting problem. I’m using the Sketchup online version. Thank you!

First suppress the materials by choosing monochrome mode, they just serve to confuse things at this stage. Then try erasing the edge between the two triangular faces. If they vanish, that’s proof that the four corners are not coplanar and can’t form a single face. You can either redraw the roof surface to make it planar or redraw then soften and smooth the edge so that the two triangles are treated as a single surface. Then apply the material to the surface.