\usepackage{amsmath}
\begin{document}
\begin{align*}
P_{loss}&=0.0015P_{1}+\0.0014P_{2}-0.0001P_{3}\\
& +0.0009P_{4}-0.0004P_{5} \label{eq}
\end{align*}
\end{document}'
Why is there an error on align? Undefined control seq?
Here you go:
\usepackage{amsmath}
\begin{document}
\begin{align*}
P_{loss}&=0.0015P_{1}+0.0014P_{2}-0.0001P_{3}\\
& +0.0009P_{4}-0.0004P_{5} \label{eq}
\end{align*}
\end{document}
You had an extra backslash in front of the 0.0014P_{2} that was messing things up; also you need to get rid of the extra blank line between the two lines within the align environment.