Please go to your Online store > Theme > Edit code > card-product.liquid file, find this line of code:
{{- 'products.product.on_sale' | t -}}
Replace with:
{%- assign percent = card_product.compare_at_price | minus: card_product.price | times: 100 | divided_by: card_product.compare_at_price | round -%}
Save {{ percent }}%