The issue is occurring maybe because of plugin or theme conflict.
You can test for conflicts. You only need to deactivate (don’t delete them) your all plugins and use a default theme (such as Twenty Twenty).
Let me know if the option is showing after this. 🙂
Thanks Animesh, I deactivated all plugins and changed the theme and the Category option has become visible. It seems the there was a fault with the theme we were using – it affected adding Categories AND commenting. Thank you 🙏