Answer
$\text{Not a polynomial function because it has a term with a negative exponent/power.}$
Work Step by Step
$\text{A polynomial function is a function of the form}$
$$f(x)=a_n x^n +a_{n-1} x^{n-1}+...+a_1 x+ a_0$$
Where the coefficients $(a_n, a_{n-1}.....)$ are real numbers and $n$ is a non-negative integer
$f \text{ can be rewritten as: }$
$$f(x)=1-x^{-1}$$
Since the term $x^{-1}$ has a negative power, the function isn't a polynomial.
