If you go to this URL and click “SearchItems” on the left-hand side menu:
You can enter your Secret Access Key ID and Access Key ID along with your Amazon associates ID and do a search. Amazon will report back a more detailed answer as to what the issue is.
If the issue is something related to “Too many requests” the issue is that you’re not driving enough sales as an Amazon affiliate to use the Product Advertising API.
Amazon implemented restrictions on their Product Advertising API which allows you to use the Product Advertising API based on the number of sales you drive as an Amazon affiliate.
Loosely speaking, the more sales you drive, the more requests you can make.
Here’s the help doc page which outlines this in more detail: