You can't delete a thread if it has been on the site for more then 4 hours or if someone has posted a response to it. You are welcome to ask a moderator if you need something deleted.
You can't change/edit a post after 4 hours of being posted, this keeps people from changing their story

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Not sure why you can't post, it would appear you have made 18 of them... Unless your attempting to make a post in the classifieds where we just changed the rules.