The Add button on the Category Picker is not visible to the Knowledge_Manager of the knowledge base. It does appear for users with the Admin and Knowledge_Admin roles so the users can create a category.
Steps to Reproduce
Impersonate a user who is the manager of a knowledge base.
Navigate to Knowledge > Articles > Create New.
In the Knowledge Base field, select a knowledge base that the user is a manager of to set the Category field to readable.
Click on the Lookup using Picker icon next to the Category field.
The Category Picker is displayed. Note that the manager is unable to create a category in the Category Picker window.
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Related Problem: PRB1248928