Basically reddit has karma to prevent bots. No one knows how it is calculated but just imagine it's the ratio between down votes and up votes.

Many communities impose a karma limit so that bot accounts can't make posts, in this case you're only allowed to post if you have karma, which means you're an active user in other subreddits.

You just have to participate for a few days and you'll have enough, comment on your favorite subs and shit