[WordPress] Display a native WordPress error message using add_settings_error.

acme-save-post.php

<?php

add_action( 'save_post', 'acme_save_post' );
function acme_save_post() {

  // Verify the user has permission to save. Code consolidated for readability.
  if ( acme_user_can_save( $post_id, 'acme_save_meta_data_nonce' ) ) {
    
    // Make sure the meta box data isn't empty
    if ( isset( $_POST['acme-input-field'] ) && 0 === strlen( trim( $_POST['acme-input-field'] ) ) ) {
      
      // Define the settings error to display
      add_settings_error(
        'invalid-email',
        '',
        'You must define a value in the meta box.',
        'error'
      );
      
    }
    
  }

}