Pandoc cannot parse equation with a fraction

Beside that $$...$$ syntax is not recommended in LaTeX, it is unclear what are you doing really.

There are not an input latex file in the command line, but guessing that is

$$
\frac{1}{2}
$$

Saving this as test.tex, then:

pandoc -f latex -t html  test.tex

Producing this output:

<p><br /><span class="math display">$$\frac{1}{2}$$</span><br /></p>

Adding -s -o test.html only will save this inside a complete XHTML file, but of course, without rendering in any way.

Guessing that you want really is render the math display class with Mathjax in a browser, then you should run:

pandoc -f latex  --mathjax  -t html test.tex -s -o test.html

firefox test.html

Output in Firefox:

Including the --mathjax flag fixed this:

The default is to render TeX math as far as possible using Unicode characters. Formulas are put inside a span with class="math", so that they may be styled differently from the surrounding text if needed. However, this gives acceptable results only for basic math, usually you will want to use --mathjax or another of the following options.

https://pandoc.org/MANUAL.html#math-rendering-in-html