How to secure aluminium window lock handle that keeps falling off?
The screw you're showing is a self-tapping metal screw.  It's designed to cut it's own channel into metal and grab hold by cutting it's own threads - but it's not meant for repeated uses because repeat uses can damage it's own threads, making its hole too big for it.

Take it to the local hardware store and get one that is slightly wider in diameter, and use it in place of the old one.