If doing what @Simin Maleki mentioned does not solve it for you, there is a chance that your default formatter is not set:

File > Preferences > Settings > Search for "default formatter" 

Make sure your Editor: Default Formatter field is not null but rather Prettier - Code formatter (esbenp.prettier-vscode) and that all the languages below are ticked. This fixed my issue.

STEP BY STEP WALKTHROUGH

Also make sure that your format on save is enabled:

1. Open settings by clicking the cog in the bottom left of the vs code side bar and selecting settings from the menu, or by hitting Ctrl+,

2. At the top right of the settings pane, hit the open file icon (if you hover, the tooltip will read 'Open Settings (JSON)'

3. Add the following line to the settings json:

   "editor.defaultFormatter": "esbenp.prettier-vscode"

In VSCode settings, search for "Editor: Default Formatter", set it to esbenp.prettier-vscode and restart VSCode.

The new way to configure prettier settings:

1. at the root of your project folder, create a new config file (I'd suggest calling it either .prettierrc.json or just .prettierrc)
2. in that new file, add json custom settings: my go-to settings for JS are as follows:

{
    "trailingComma": "none",
    "tabWidth": 4,
    "semi": true,
    "singleQuote": true
}

I'd suggest doing this in each of your projects and including in any source control, that way every pull of the repo will automatically set some base settings for that developer's instance of prettier.