If you read the wiki, %category% might not always work.
Note on using %category%
%category% does not work correctly with some implementations of mod_rewrite in Apache versions prior to 2. If you are using Apache 1 and experience problems with using %category, either do not use %category% in your permalink structure, or refer to Schlueterica’s plugin (https://isaacschlueter.com/plugins/i-made/lucky-13-rewrite/).
Note on using %category% with multiple categories on a post
Where you assign multiple categories to a post, only 1 can show up in the permalink. This will be the lowest numbered category (see Manage Categories). The post will still be accessible through all the categories as normal.
