The reason has been stated above, and is derived from the change that took place here: Enhanced newbie restrictions & requirements.

You’d need to either:
-   Rank-up to Jr. Member (30 Activity; 1 Merit at least).
-   Purchase a Copper Membership.
-   Ask in Meta (or move this thread there) in order to see if there is a way for Admins/Mods to clear a signature in cases such as this.