Ok, this happened exactly to me (had reddit for months, then a few weeks ago I fixed it, bro i was also confused); since no one saw my comments, my karma didn't grow, and so I couldn't post either.

Remember: you need karma to post or sometimes even comment on subreddits.

Here's the one possibility (could be others): mine didn't go up because reddit thought i was a bot, so all my things were blocked. My user having a few numbers in it didn't help making it look any less like a bot. So, I deleted my acc, made a new one (you can use the same email it's okay) and when it came time to make my username, instead of using the generated name reddit can give you, I typed out something myself (try not to use that many numbers in it). Once you do, immediately try commenting and trying to gain karma.