@thirumalmurugan, Unfortunately we are not the author of that payment option , PayPal standard comes with woo-commerece. Generally security headers not valid error comes due to wrong paypal API and credentials. I think you should check those and make sure to add correct one.
Also, I would recommend that you switch to our plugin, at least for Express Checkout. This will give you a lot more features and functionality than the built in PayPal Standard provides, including the ability to add PayPal buttons to your product pages and cart page so that people can skip the WC checkout page. This leads to increased conversion rates. It also includes PayPal Marketing Solutions functionality which can further lead to increased conversion rates and also increased avg. order totals. Also more options for refunds and many other gateways as compared to Standard paypal.
Let me know if you have any other questions or concerns.
Thanks!
