Why government cannot print new currency to pay the debts? 

When there is deficiency of internal resources then government borrow. Government can borrow either from central bank (internal sources) or from IMF and World Bank (external sources). But when government borrow from central bank by printing latest notes, the quantity of money in circulation enhances, which leads to enhance in aggregate demand and then in inflation. So, Government has to borrow from external sources, which is another way to solve the queries.