Hello,
On the checkout page, you can make only 3 optional fields (Company name, Address line 2, Phone field), in order to do it, go to Appearance -> Customize -> WooCommerce -> Checkout.
But if you want to make “Last name” optional, you need to download the “Checkout Fields Manager for WooCommerce” plugin, then go to WooCommerce -> Checkout -> select the “Billing” tab -> Click “Add New Field” -> and there enter some “label name” and select the type of your option field
https://monosnap.com/file/c4hjZwQQ045xAjp0GPI8sgw4VlG1OK
https://monosnap.com/file/uyMyww6IqhFM6E7TC4Ep6vrv33KtMq
https://gyazo.com/7e28087fbbd6da878e22a8374d19c9ac
Here is a description of the plugin:
https://wordpress.org/plugins/woocommerce-checkout-manager/#description
Kind Regards
.
