How do I fix this and why is it happening?

You either have a cracked account or you use the demo, if you use some sort of alt thing on a client then you don't have 20 dollars

Could be cracked. This happens with quite a lot of people if not careful. It's pretty easy tapping into that stuff these days.

Either try resetting your router, re-installing minecraft, or maybe checking to see if your firewall is blocking it. If you're using an unofficial minecraft software (pirated, or just taken off another website besides the official one) whether you payed for it or not, the possibilities of the program being a fake is high. There's always a flaw in every pirated game cause they kinda have to make it that way to work.

Check the internet and if you typed in the server correctly and the server is up and running. Check the minecraft forums for more help if this doesn't work.