Hi guys. I wanted to advertise myself as a possible contractor and showcase my skills. I can see others have done this, but when I try to post it, there is a required field at the bottom saying that I need to choose a studio/company developer or indie developer option to post my ad.

I am neither - I am just a person offering my skills to anyone that could need or want me on their project. Am I missing something here, or the payment is necessary to limit spam classifieds and non-serious contractors etc?

Just curious. Thanks, guys!

Just put "none" and you'll be fine. It is required because most people look for others with previous work history.