Okay, so GameMaker is telling you that you are using a variable which hasn't been declared yet, and so doesn't actually exist. In this case it's saying that the variable Background is being used by the obj_Player when calling the function layer_get_y(), but it hasn't been defined with any value yet. Great! Now you know what the issue actually is but how to solve it? Have a look at the manual for the function layer_get_y():
Check what the text says about the input parameter for the layer: "You supply the layer ID (which you get when you create the layer using layer_create()) or the layer name (as a string)"
So, the issue here is that you are using the layer name to target without telling GameMaker it's string, so it thinks that Background is a variable when it should actually be "Background", a string.