Please tell us more about your environment, especially the Moodle version, and what plugins have been upgraded.
Do you have records in this table? If you have database access you can check this with SELECT * FROM mdl_course_categories.
Also check the default course category for the site which can be done with SELECT name, value FROM mdl_config WHERE name = 'defaultrequestcategory'. Check that there is a row present in mdl_course_categories with an id matching that shown for defaultrequestcategory.