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There is no public function called add_cap() - just the class methods inside WP_Roles, WP_Role, WP_User that can add capabilities.
This page was moved to https://developer.wordpress.org/reference/classes/wp_role/add_cap/, https://developer.wordpress.org/reference/classes/wp_roles/add_cap/ and https://developer.wordpress.org/reference/classes/wp_user/add_cap/ except above language locator.