the_category if/else statement

Is the code used inside of the loop? The following code should work:

if ( class_exists( 'DPortfolio' ) ) {
      echo DPortfolio::instance()->dportfolio_get_category();
}else{
    the_category();
    //or echo get_the_category();
}

If outside of the loop you can try echo get_the_category( $post_id ) to show the categories.

Note: As you are getting an error, if you edit the question, include the full code snippet with if/else and include the error, then it’s easier to help.