Added errors documentation for paypal express token mutation (#8457)

Co-authored-by: Kevin Harper <[email protected]>

Author: sathiyaaa

src/guides/v2.3/graphql/mutations/create-paypal-express-token.md

@@ -112,3 +112,14 @@ Attribute |  Data Type | Description
 --- | --- | ---
 `edit` | String | The PayPal URL that allows the buyer to edit their checkout details
 `start` | String | The URL to the PayPal login page
+
+## Errors
+
+Error | Description
+--- | ---
+`Required parameter "cart_id" is missing` | The mutation does not contain a `cart_id` argument.
+`Could not find a cart with ID "XXX"` | The specified `cart_id` value does not exist in the `quote_id_mask` table.
+`Field PaypalExpressTokenInput.code of required type String! was not provided.` | The required attribute `code` is missing.
+`The requested Payment Method is not available.` | The payment method is not configured.
+`Field PaypalExpressUrlsInput.cancel_url of required type String! was not provided.` | The required attribute `cancel_url` is missing.
+`Field PaypalExpressUrlsInput.return_url of required type String! was not provided.` | The required attribute `return_url` is missing.