Having the same problem I went down to the database.
with this SQL you can update ALL categories to noindex and to nofollow for search engines (google, bing, yandex)
UPDATE `jos_categories` SET `metadata`='{"author":"","robots":"noindex, nofollow"}' WHERE `extension`='com_content'
you need to find out your table prefix, in the above example jos_ is the default prefix of joomla. If your table prefix is xzhw_ of course your SQL would be
UPDATE `xzhw_categories` SET `metadata`='{"author":"","robots":"noindex, nofollow"}' WHERE `extension`='com_content'
be careful
if you use categories for displaying other content in e.g. a category blog, DO NOT USE THE SQL. Instead you should eiher select your categories in the WHERE clause or even better you go manually through the categories in the backend of joomla and you edit under Publish-tab the Robots to noindex, nofollow