How can I allow the users to place an order only from a particular state?

To allow users to place orders from a state by enabling delivery pin code/area for the particular state.
You first have to enable the delivery pin code/area from Settings > Shipping settings in StoreHippo admin panel.
Then go to Settings > Delivery availability section in the admin panel and add the pin code or area of the particular state.

For complete procedure, please refer:
https://help.storehippo.com/topic/delivery-availability