I've created post labels in social studio but the same is not reflecting in label section while posting a new post on my facebook page.I am not able to figure out where I'm doing wrong.
What you're creating are post labels and are used in engage section of social studio. These labels can be used while replying any comment or inbox messages etc. or putting labels on existing comments or inbox messages.
If you want to use labels while posting anything new on your social account page, create publish labels. These labels can be found at overall account level in admin section.
Hope this help.