I got a good answer and tried to upvote and a red window popped up that said something like:
must have 15 points
Why is this? Have I done something wrong?
PS: Red pop-up windows...too many!
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The best way to thank the user who posted the answer is to mark the answer as accepted. It gives them a big rep bonus (15 reputation, compared to 10 reputation from an upvote). It also acts as a signpost saying: "this is the answer that solved my problem."
Apart of that you can just earn 15 rep (it really is not that hard) and then add an upvote.
Some users post contact details and "wish lists" on their profile, if you want you can check for that as well.
Upvoting is the main way of thanking people for their answers.
I think there are two main reasons why upvoting is not open to anyone, but requires a little reputation. One is that votes are supposed to indicate that an answer is useful, which implies that the voter is competent to judge; we require voters to demonstrate their competence actively by participating in the site, i.e. by asking or answering a few questions, with competence validated by upvotes to the participants. Another reason is to prevent fraud: otherwise it would be extremely easy to create many accounts and vote with all of them.
15 reputation means two upvotes on an answer or a question. With this question alone, you've reached the threshold on this meta site.
Another important way of thanking answerers is that if someone posts a satisfactory answer to a question that you asked, you should mark their answer as accepted. The asker is uniquely able to determine which answer helped him best, and posting a question constitutes involvement on the site, so you can always accept an answer, with no reputation requirement. You get a small reputation bonus for doing it. Usually, you should upvote all the answers that helped you somewhat, and select the best one (by whichever criteria you like) as accepted. But don't mark an answer as accepted if none of the answers are good — marking an answer as accepted tends to convey the message “I'm satisfied with the response, I'm not looking for answers anymore”.