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}
