Depending on what post editor you are using:
- Classic – make sure to switch the editor to “Text” instead of “Visual”. The latter will output anything you write in a form of text, while the first one will process the shortcode.
- Gutenberg – find a plus sign in the top-left area of your WordPress dashboard and add a “Shortcode” block.
If you add the shortcode manually to the file, make sure it’s before the closing PHP tag: ?>