Hi Sinha,
Check your Business User login which has all access rights for the system and for the adaptation. Now as you mentioned it was checked.
Remove the Partner development WC access.
But my sincere advice is to not to take chance with a single user for both the development and the same user as Key user, sometimes it may work and sometimes it may not work . So better to use an id offs key user or a business user and try to create. It will surely allow you to create a field.
Regards
Vignesh Ravichandran