The map is just full. You just need to keep trying. c: You can always check how many players are in a map by hovering your mouse over to the right of the page.
Don't worry this happens to everyone, mostly happens when trying to get into Fluorite Plains, because it's such a popular map. Everytime it happens to me, I just keep retrying and retrying until it finally lets me in. All you need is patience. Getting on at an earlier time will give you a better chance of getting in, too.
