You have two options to overcome this issue.
1. Use an OpenType Math Font
For unicode-math
to work you have to use an OpenType font which provides the math table, such as Latin Modern Math
. This unfortunately defeats the implicit purpose of your MWE to use a sans serif font for maths.
\documentclass{article}
\pagestyle{empty}
\usepackage{unicode-math}
\setmathfont{Latin Modern Math}
\begin{document}
Lorem ipsum
\[ \sum_{k=0}^\infty a_0q^k = \lim_{n\to\infty}\sum_{k=0}^n a_0q^k =
\lim_{n\to\infty} a_0\frac{1-q^{n+1}}{1-q} = \frac{a_0}{1-q} \]
dolor sit amet
\end{document}

2. Use the mathspec Package
The mathspec
allows to use any OpenType font for mathmode (at the cost of bad spacing).
\documentclass{article}
\usepackage{mathspec}
\setmathfont(Latin,Digits,Greek){Latin Modern Sans}
\setmathrm{Latin Modern Sans}
\begin{document}
Lorem ipsum
\[ \sum_{k=0}^\infty a_0q^k = \lim_{n\to\infty}\sum_{k=0}^n a_0q^k =
\lim_{n\to\infty} a_0\frac{1-q^{n+1}}{1-q} = \frac{a_0}{1-q} \]
dolor sit amet
\end{document}
