If you post the exact wording of the ad, we can tell you what is causing the problem. When it looks like you wrote an ad.... but the ad never shows up on the site.... it is called "ghosting". Here are some things that can trigger it:
Putting L@@K or !!!!!!! in the title.
Not putting a price in the price box.
Posting HTML in the ad (like you'd see on a website).
Posting more than 1 ad every 48 hours.
Posting huge photos that won't show up on a smartphone.
More than likely it's one of two things. Either it's a glitch in the system (wait a few days and try again) or you are breaking the TOS and they finally caught you and won't let you post.
Best to ask and provide all the facts in the craigs list posting help forum on craig's list.
could it be what you are advertising?
I made a couple ads on my account and when I clicked 'published', I get the screen that says it's published and I should receive an email. But when I go back onto my account page, it wasn't really published at all, but saved as a draft. Plus, I went to repost an older ad, it said it was published. Again, I went back to my list of ads and the status still didn't change and it was still listed as an older ad. I have no idea what's going on, I've been using Craigslist for years and never had this issue. Now I can't post anything because the 'published' ads are going to my draft folder.