My MacBook stays open when I close the lid

For a test you need a magnet. Then if you move it from back to front on the left edge, it should if working cause the screen to go dark when you get to the sleep control. If that does not happen there may be a hardware problem. You can also check to see if the magnet is still behind the screen bezel on the left with the same magnet. Do you have internet sharing on in system preferences sharing. Or any sleep programs installed like insomniac. These could cause the problem. You could use terminal to check also. read this link.https://www.guidingtech.com/47697/macbook-sleep-issues/

there is a magnet that is on the left side of your screen. It's a little more than half way down that interacts with an eclectic control piece inside your MacBook that causes the screen to go to sleep did you have the front bezel for the screen off. Otewise you may have something not connected inside on the left side by you USB ports.

Since what I have is a 2010 MacBook, I could be wrong about the location of the sensor that causes sleep with lid closed. It could be on the other side by the DVD drive. I haven't had an earlier MacBook in some years. What I do know is that the sensor is built into the logic board. At least on mine it is. if you find it there and it works then you would want to know if the magnet in the lid is in place.