How can I write a ° (degree) symbol in LaTeX?

The \degree command is provided by the gensymb package, so if you add:

\usepackage{gensymb}

to your preamble, that should enable the command.

Another alternative is the \textdegree command, which is provided by the textcomp package. And, finally, $^{\circ}$ is another way of obtaining roughly the right symbol.