I was dealing with a similar issue recently. The problem was that my AWS account was signed up for FPS, but not the FPS Sandbox. And I was trying out my keys with the sandbox test urls.
Not sure if this will solve your problem, but please make sure that your AWS account is signed up for both the "FPS" service as well as the "FPS Sandbox" (https://payments.amazon.com/help/Amazon-Flexible-Payments-Service/Technical-Resources/Amazon-FPS-Sandbox).
Once you're sure that your AWS account is registered for both, make sure that you change the URLs to the sandbox or production environment appropriately; both when requesting the CBUI pipeline and when performing action of the Amazon Service token.
Hope this helps,
S.