According to the law of conservation of mass,mass can neither be created nor be destroyed.so why every equation is not a balanced equation?for...

I believe you are talking about balancing of chemical
equation. When a chemical equation is not balanced it means that that is is not
representing the reality correctly. An incorrect equation may or may not conform the law
of conservation of mass. It makes no sense in drawing conclusion about validity of law
of conservation of mass or of any other law based on an incorrect
equation.

Taking specifically the chemical equation
referred in the question, that is

Mg + O2 -->
MgO

The equation is incorrect because it implies that 1
atom of Mg combining with 2 atoms of oxygen will produce 1 molecule of MgO, which
contains 1 atom of magnesium (Mg) and 1 atom of oxygen (O). Thus the left hand side of
equation has 2 atoms of oxygen while right hand side has only 1 atom of oxygen.
Obviously this equation is incorrect. To make the equation correct, we will need to
balance it in such a way that atoms of each of magnesium and oxygen are same on both the
side. We do this by changing the equation to:

2Mg + O2
--> 2MgO

Please note that when a chemical reaction
is represented in the form of a chemical equation, the equation must be balanced. An
imbalanced chemical equation is as improper and incorrect as word written with wrong
spelling or a sentence with incorrect grammar.Imbalanced equations should not be used of
representing chemical reactions.