Your AccessKey Id is not registered for Product Advertising API
Sometimes, you may get the error "Your AccessKey Id is not registered for Product Advertising API", even if you are using the correct Access Key Id and Secret Access Key.
This can be because of a temporary glitch in the Amazon system.
How to fix this:
First of all, do not panic. Everything is ok. :-)
Wait for 24-48 hours and try again.
If this doesn't work, you may need to DELETE the ROOT Access Key ID and generate a NEW Access Key ID and Secret Access Key, as shown here.
Please note that you need to use the new Access Key ID and Secret Access Key TOGETHER. Don't just generate a new Access Key ID and use that instead of the old one. :-)
If the issue still isn't fixed, you will need to contact Amazon associate support to see if they need to unblock something in your account (this happens quite often).