My first coding - Error - A document must be saved with an extension (and without spaces or accents in the name) before being used by a command

Question

\documentclass{article}

\title{My Equation}
\author{First Last}
\date{July 21, 2020}

\begin{document}
\maketitle


\begin{equation}
    K = P_{predicted} H^{T} \Big( H P_{predicted} H^{T}+R\Big) ^{-1}
    \label{eqn:KalmanUpdate}
\end{equation}

\end{document}