Cannot add category on Periodic Account Assignments

Hi all, I have one issue with the form of Periodic Account Assignments. I want to add category on MTL Accounts section, then NEW button is unavailable and I couldn't add. Moreover, when I use field of old Category code changing New category code, it wasn't saved and updated even I click on SAVE. Please help to add new category on this form. Or is there any specific requirements/steps to do before this action? Thank you!