LaTeX doesn't show negative sign in exponent

The problem occurs because the symbol − that's used in both exponents is not a "normal" dash symbol, -. Instead, it is the Unicode symbol U+2212, the "math minus" symbol. (Did you maybe copy and paste the input from some other source?)

Here are three remedies:

• Switch to either XeLaTeX or LuaLaTeX and load either the fontspec package or the unicode-math package (which loads fontspec automatically).

• If you need to use pdfLaTeX, add the following instructions to your preamble:

  \usepackage[utf8]{inputenc}
  \DeclareUnicodeCharacter{2212}{-}
  

• Finally, consider simply replacing all instances of − with -. (That's what's done in the code below.)

\documentclass{article}

\begin{document}

\section*{Your code}
The weight decay was set to $1 \times {10^{−4}}$ for Model 8 and $5 
\times 10^{−4}$.

\bigskip

\section*{Correct code}
The weight decay was set to $1 \times {10^{-4}}$ for Model 8 and $5 
\times 10^{-4}$.

\end{document}

I fixed your codes as follows:

The weight decay was set to $1 \times {10^{-4}}$ for Model 8 and $5\times 10^{-4}$.

And the following is the PDF style.

The reason that why your − can't be complied may be that you type it not in English typewriting. Please make sure you are under English typewriting when you type codes in math environment.