Customer account creation is managed by the ecommerce platform and will depend on what platform your store is using. 

Shopify Checkout Integration or Migrated Shopify Checkout Integration

On the Shopify Checkout Integration, you can force customer accounts at checkout from your Shopify settings. See Shopify's Enabling or disabling customer accounts guide for more information. 

Recharge Checkout on BigCommerce

Login and account creation at checkout is not supported on the Recharge Checkout on BigCommerce. If the customer logs in before checkout, Recharge auto-populates the shipping and email address into the checkout fields for the customer.