I believe that signatures are one of the "advanced" features that are not enabled until you meet certain thresholds. Those include 100 posts, at least 30 days of membership, and some number of reputation points; I don't know how many. The signature feature will be automatically enabled when you meet those requirements. Until then, you can be creative with your member title
(Also, a friendly reminder to anyone reading this: we do not allow "fluff" posts in order to boost your post count and access advanced features. If we detect such posts, we'll give you infractions for them.)