Whenever I am going to write an answer or edit someone's post, I am not able to see its preview under the answer writing section in MSE. Why is this happening?
This is the (deprecated) mobile web version of the site; it does not have a preview when editing posts.
To switch back to the full version, click the 'full site' link in the footer. It seems you have a large enough screen to not need using the mobile web version.