On Live Agent Pre-Chat in Chat Setting page I am not able to add Description from the Case. I can add other field from the Case why isn't in the selection?
The out-of-the-box pre chat form has certain limitations and doesn't support all fields and features. The workaround for this is to use a custom aura / web components. We had to take this approach as the standard pre-chat form doesn't support picklist dependencies and we needed to have a dependent picklist on our form for case categorization.
Here's some documentation around it : https://developer.salesforce.com/docs/atlas.en-us.226.0.snapins_web_dev.meta/snapins_web_dev/snapins_web_lightning_components.htm
Hope this is helpful.