Weird wall texture when painting??

I was working on finally flipping the family house to sell because I wanna live in the old factory house, but suddenly after I had built a wall, I tried to paint it and it didn't blend in. It looked more like I was painting on dry wall. I tried pressing 'r' to change the paint roller thing in the corner, because I know there's wall textures there from building in the sandbox mode... but it didn't work. All it did was adjust the size of my paint roller.. so Im not sure what to do.

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