I can't comment yet so here's my best try at an answer:
Prettier mainly supports these languages according to their page: JavaScript · TypeScript · Flow · JSX · JSON CSS · SCSS · Less HTML · Vue · Angular GraphQL · Markdown · YAML
Make sure you're formatting one of those languages for best results.
If Editor: Format On Save Mode
is set to modifications
then you may not be able to go to an existing file and instantly format all of it.
If none of that helps I'm stumped and recommend looking at Why Prettier does not format code in VSCODE? if you haven't already
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