In $\frac{p}{q}$ form of rational number why $q$ is not equal to $0$.

Every rational number in the form $( \frac{p}{q})$ can be expressed as either a terminating decimal or a non-terminating and recurring decimal.

If $q=0$ then $( \frac{p}{q})$ neither terminating nor recurring.

So $q$ is not equal to $0$.