Ok, I figured it out by properly registering for the Amazon API promotion, I thought I had already done that. Sorry about that.
I mistakenly ignored that at one point I received this error:
Error: Your AccessKey Id is not registered for Product Advertising API. Please use the AccessKey Id obtained after registering at affiliate-program.amazon.com/gp/flex/adv...ing/api/sign-in.html
Go there even if you think you have already done that!