This article also answers:
I am unable to configure Amazon Payments/Go Cardless with my Chargebee site. What is the reason?
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If you see an error "Sorry, we couldn't find that page. Maybe it was moved, or you accidentally mistyped the URL", when configuring PayPal Express Checkout, it means you're using a non-compatible theme with the V2 hosted pages of your Chargebee site.PayPal Express Checkout works only with the Minimal, Elegant or Inspire themes. With Chargebee's Rhapsody theme, it doesn't.
In case you're using the Rhapsody theme, just change it under Settings >> Checkout & Self-Serve Portal (under Customer-facing essentials) >> Hosted Pages Settings >> Themes and then try configuring PayPal Express Checkout.
This applies to GoCardless and Amazon Payments as well.