documentclass{beamer}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage{lmodern}
usefonttheme{serif}
usetheme[progressbar=foot]{metropolis}

usefonttheme{serif}

! Package inputenc Error: Unicode char ₂ (U+2082)
(inputenc)                not set up for use with LaTeX.

documentclass{beamer}
usetheme[progressbar=foot]{metropolis}

begin{document}

begin{frame}
frametitle{Test}

This has a subscript₂
end{frame}

end{document}

documentclass{beamer}
usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usetheme[progressbar=foot]{metropolis}
usepackage{FiraSans}

DeclareUnicodeCharacter{2082}{textsubscript{2}}

begin{document}

begin{frame}
frametitle{Test}

This has a subscript₂
end{frame}

end{document}