Questions

'waiver' should be used to make the sentence correct.

The error is in third part. By using ‘not’ after ‘unless’, both the negatives cancel each other. So, the correct usage will be ‘unless he reports for duty immediately’. Since “unless” is negative it should be
followed by affirmation, like “unless he reports for duty”. Incase “unless” was absent, we could have written “if he does not report” and expressed the same meaning.

‘prefer’ is a verb that is always followed by the preposition ‘to’ expressing choice or preference over something else. In this case choice of ‘this book’ over the ‘other’ one. ‘Than’ is used for comparing to options, ‘prefer to’ expresses choice. Correct expression will be ‘to that one’.

GST  is  a  destination-based  tax,  it means   end   users   consuming   any goods or services is liable to pay the Goods  and  Services  Tax.  The  tax  is collected  by  the  State  in  which  the goods or services are consumed and not  by  the  state  in  which  goods  are manufactured    and    If    there    are exports,  the  seller  of  the  goods  or services  is  exempted  from  paying the tax. GST is a consumption-based tax,  it  means  the  state  where  the goods  were  consumed  will  receive GST   and   where   goods   were   sold should  not  get  any  taxes.GST  is  a value added tax as it is based on the increase  in  value  of  a  product  or service  at  each  stage  of  production or   distribution.GST   is   not   gross value tax.