You’re currently in a player corp, and you’re required to be in an NPC corp for posting a sale:
As per CCP’s character sales rules, which you can review here Welcome to the Character Bazaar - Marketplace / Character Bazaar - EVE Online Forums you’ll note:
The character being sold must be in an NPC corporation at the time of posting, and for the entire duration of the sale/auction thread being active.
Section B, Para 2.