documentclass[12pt,letterpaper]{article}

usepackage{amsmath}
usepackage{amsthm}
usepackage{amsfonts}
usepackage{amssymb}

begin{document}

section{Hello World}
    begin{subequations}
        frac{a}{b} = 0 \
    end{subequations}

end{document}

line 12: Missing $ inserted frac{a}{b}
line 12: Extra }, or forgotten $ frac{a}{b}
line 13: Missing $ inserted end{subequations}
line 13: Missing } inserted end{subequations}