Var_dump ACF “API”

Probably nowhere because it outputs too early. I recommend to use error_log function and turn on WP_DEBUG_LOG.

Just add to the wp-config.php file somewhere in front of this line:

require_once(ABSPATH . 'wp-settings.php');

these new lines:

define( 'WP_DEBUG', true );
define( 'WP_DEBUG_LOG', true );

Now create in wp-content folder a new file debug.log and set chmod to 640.

And this line in your code:

var_dump($post);

replace with this:

error_log( var_export( $post, true ) );

Now you will find content of the $post variable in the debug.log file.