Hi,
The display of the price per weight unit is only available on the product details page not on the listings.
Philipe thought you had configured several prices on your product, with a different minimum quantity in each price and you wanted to display the one for the unit price (when the quantity is 1) instead of the cheapest as it is set by default.
To do what you want, you'll have to edit the file product / listing_price.php via the menu Display>Views. There, you want to change the code:
if($this->params->get('show_price_weight')){
if(!empty($this->element->product_id) && isset($this->row->product_weight) && bccomp($this->row->product_weight, 0, 3)) {
to:
if(true){
if(isset($this->row->product_weight) && bccomp($this->row->product_weight, 0, 3)) {