BGGET gets a character of a background layer, so you would need
layer to check with
x coordinate
y coordinate
whether you want to read it in pixel coords or character coords (1)
You need the
(layer, the middle x of the screen or the player X variable, the middle y of the screen or {the player y variable minus 19}, 1)
notice how there are only 3 commas. That means there are ONLY four things you can put into BGGET. And they should be these four things.
But I won't help you on the last stretch, sorry. It's not that big of a deal that you want me to help a bit more. But the fact that you would only give me credit for FULLY implementing it into your code after spending an hour and a half for making a working example is kind of sad.
If you feel overwhelmed by my code, then just edit mine. I'm sure it wouldn't be too much of a change.