I need a shortcode or snippet to display the product primary category

Hello,

Thank you for contacting Rank Math and bringing your concern to our attention.

Unfortunately, we don’t have a variable for getting the primary category of the product.

However, you can use this filter to create a new variable on your site which displays the selected primary category:

add_action( 'rank_math/vars/register_extra_replacements', function(){
	rank_math_register_var_replacement(
	   'my_primary_term', [
	   'name'        => esc_html__( 'Custom variable name.', 'rank-math' ),
	   'description' => esc_html__( 'Custom variable description.', 'rank-math' ),
	   'variable'    => 'my_primary_term', /* the actual variable: %my_primary_term% */
	   'example'     => my_primary_term_call_back(),
	   ],
	   'my_primary_term_call_back'
   );
});
function my_primary_term_call_back(){
	global $post;
	return get_cat_name(get_post_meta( $post->ID, 'rank_math_primary_category', true ));
}

Here’s a guide on how to add a filter to your site:
https://rankmath.com/kb/wordpress-hooks-actions-filters/

Once done, you can use this variable %my_primary_term%to retrieve the primary term.

Hope that helps.

Thank you.