Re: Not able to make new posts – database error
In phpMyAdmin you can see what the next autoincrement value is going to be for post_id. I would look at the posts and order them by post_id to see the highest, then I would see if the next autoincrement value is going to conflict with that. If it is, just reset the autoincrement value to something higher, even if you skip some values.
You need something like phpMyAdmin or command-line mysql to take care of this. Do you have access to something like that?