When I contacted PayPal regarding PayPal Express Checkout not showing a Guest Checkout option to customer they said this :
Thank you for contacting PayPal Merchant Technical Support.
I understand that you have questions about why Guest Checkout is not appearing on your checkout.
The issue has to do with how the payments are processing with Shopify. Shopify has started sending this API value “MerchantInitiatedBillingSingleAgreement” in the payment requests. This value creates a billing agreement with your website and the buyer. This agreement requires the buyer to log into a PayPal account or to create an account with PayPal. This is a new process that Shopify implemented. It is to streamline the checkout process for returning buyers to your website.
Shopify hosts the code for their shopping cart and website, so you may not have made any changes to your integration, however, Shopify has made updates to their shopping cart. These updates are removing Guest Checkout through PayPal as a payment option.
I apologize for the inconvenience this issue has caused.
Thank you for using PayPal and have a wonderful day!
I would be glad If anyone can help me regarding this issue or what does it mean.