Price per unit is a Shopify admin feature that is currently only available for merchants located in France or Germany - you can read more about this feature in Shopify's documents here:
Currently, our Flow themes have 'Price per unit' support installed as standard to allow the 'Price per unit' measurement to be displayed on the following pages:
Cart page & drawer
Collection list page
You can see an example of the functionality here:
To enable this function on your Shopify Product on Flow, please ensure that the following factors are all correct for your storefront:
A Store location set somewhere in France or Germany.
'Show unit price for this product' is enabled for the Product.
Double-check a price and measurement have been assigned.
The Product has no variants enabled.