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

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}+RBig) ^{-1}
    label{eqn:KalmanUpdate}
end{equation}

end{document}