Why can I not change the primary poster of my own film?!

I need to change the poster from what's currently there to the new image I uploaded the other day, but get this message: "The existing primary has been provided by a studio partner and is not editable." I am the filmmaker, so I have no idea why I'm getting this message. Any help would be greatly appreciated! Thank you! https://www.imdb.com/title/tt27791072/