I set a background on a 2d sidescroller(giving it a negative y value) but my character can stand on it even though he has a y value of 0

I set a background on a 2d sidescroller(giving it a negative y value) but my character can stand on it even though he has a y value of 0, also when im trying to place the player start in front of the background sprite it says “BADsize”.
i used these tutorials series to build the game: https://www.youtube.com/watch?v=zqkN_IIheRY
please help!