I just checked your website code and it looks like the strikethrough price is not coded to show in the product page so you would have to manually add the code of the strikethrough price to show in the product page. Of course, it will require some changes in your theme files, most probably in main-product.liquid so if you’re not familiar with coding then I would highly recommend you to hire a Shopify developer who can help you out with that. Feel free to connect with me if you want me to help you out and I would be more than happy to give you a helping hand.