Could you please check how it works with some default WordPress themes like TwentyTwenty or WooCommerce Storefront to understand is it our theme issue or not?
Could you please elaborate your concern more with some useful Screenshots or small video of the issue so that we can better understand and assist you accordingly.
Your issue has been resolved. under the Dashboard > Layouts > there was a Custom Layout enabled for the checkout page which is why you weren’t able to see the sendcloud button. So, for the time being we have disabled the checkout layout and now the button shows fine on your Site.
Please check back your Site after completely clearing the browser cache.