I approved an answer here yet I didn't receive two rep for doing so. Why is that?
That's because it's a question on Meta, and votes and accepts don't influence your reputation. It's simply copied (once an hour or so) from the main site. See the Help Center article What is "meta"? How does it work?:
Votes on meta do not affect your reputation; your meta reputation is the same as your reputation on CS50 Stack Exchange (synchronized hourly), though you earn separate badges. You must have 5 reputation to participate on meta.