In default setting, category slug is not displayed on single post.
To override this behaviour, add following snippet into your theme functions.
Appearace > Editor > functions.php
Hello everyone, we are having a week off from June 26, 2017 until July 2nd, 2017 due to national holiday. We will have a support officer standby in between but won't be able to answer in the normal pace. We hope for your understanding. Thank you.