Hi,
Have you tried ScratchPad to test your keys? If you get the same message with ScratchPad, it’s not specific to the plugin.
Regarding the error message, please see what Amazon says about it.
Effective 23-Jan-2019, the request limit for each account is calculated based on revenue performance attributed to calls to the Product Advertising API (PA API) during the last 30 days.
Each account used for Product Advertising API is allowed an initial usage limit of 8640 requests per day (TPD) subject to a maximum of 1 request per second (TPS). Your account will receive an additional 1 TPD for every 5 cents or 1 TPS (up to a maximum of 10) for every $4320 of shipped item revenue generated via the use of Product Advertising API for shipments in the last 30 days.
Note that your account will lose access to Product Advertising API if it has not generated referring sales using PA API in the last 30 days.
source: Troubleshooting Applications
That particular error message can be observed when you make many Amazon Product Advertisement API requests in a short period of time, reaching the rate limit set by the Amazon side. One way to workaround is to set a longer cache duration in the unit editing page and make actual referral sales.
For questions regarding the Amazon’s guideline, it’s best to ask the Amazon support.