How to apply specfic coupon in ubercart while creating an order from backend means admin pages
1.) How to add coupon codes on add order admin page ? i need to add orders manually from backend but not able to apply coupon codes for
  discounts, there is only a feature to "add a line item" but not for
  coupon. I think there should be a textbox to fill coupon code and
  apply button to apply it.


If you want to apply a coupon code from the admin page. Use the uc_discounts_alt module that provides you three submodules(uc_discounts,codeless_discounts_field,product_price_alterer_field).
Enable the uc_discounts module it will provide you the textarea at the admin configuration page to apply the discount code.
1)Go to admin >> store >> orders >> create
2)Enter the customer information
3)It will open the create order screen as shown in the screenshot below 


  2.) If coupon is applied to manually created order then tax calculation should be done after coupon price deducted from subtotal.


Yes.