Show Yoast Primary Cat

// Add to function.php

function taxo_primary_term_name($taxo){
  $wpseo_primary_term = new WPSEO_Primary_Term($taxo, get_the_ID());
  $wpseo_primary_term = $wpseo_primary_term->get_primary_term();
  $cat = get_the_category(); 

	if ( $wpseo_primary_term ) { // Return yoast primary category
		return $wpseo_primary_term = get_term($wpseo_primary_term)->name;
	}
	else { // Otherwise reture the first normal category
		// Return the first term.
		return $cat[0]->cat_name;
	}
}

// Put in themplate where you want it to show up
<?php echo taxo_primary_term_name('category'); ?>