I am trying to join the sig campaign of 777, but I keep getting error "last post was ... this much ago" something like that
This is because you are posting to fast, once your rank goes up you can post faster. The forum has different rankings, and payments are different based on it.
They are : brand new, newbie, jr. member, member, full member, senior member and hero member. Once you are jr. member you won't have any problems with waiting time