Security Header Not Valid -PayPal

Apologese if this is a duplicate post, I don't think the original was sent.
v 3.0.3.8
After going through chekout and clicking PayPal there is an error " Security Header Not Valid " see attached screen grab.
You can try the error at www.rusticwoodcraft.co.uk if it helps.
Any ideas what is wrong, me or PayPal.
Thanks
Roy

You don't say which PayPal extension you are using, but it could be your PayPal credentials have been entered incorrectly in the extension.

Try Paypal Commerce from the Opencart marketplace.

BTW.: You got a number of other issues, too, on your website. Such as wrong placement (e.g. not popup) of the cookie consent. And some missing icons when using the 'www.' in the URL.