Scratch

when gf clicked
if <touching [solid sprite v]>
move [-1 steps]

Um I don't really know how to do the blocks on the forum... The "touching __" block should be light blue and the move block should be dark blue, and "solid sprite" you pick from the "touching" block's dropdown (and instead of "solid sprite" it'll be the name of the sprite that you can't walk through e_e)

If it isn't a sprite, but rather a certain color that the character shouldn't be able to walk through, use the "touching (color)" block instead of "touching (sprite)." This is very irritating to explain when I can't use pictures. I could make a project to demonstrate if you want.