If you receive the error message „The partner tag is not mapped to a valid associate store with your access key„, this means that the Associate ID (partner tag) used for the Amazon PA API request is not listed in your Associate Account for that store.
This behavior is new since PA API 5.0! In the past (with PA API 4) it didn’t matter with which partner tag the API was requested. PA API 5.0 checks the connection between Associate Account and partner tag.
If your Associate Account is registered for the German and English Store and you have set your partner tag of the German Store as default in ASA2’s setup, requests to the English Store won’t work without further ado with PA API 5.0.
So that requests to other stores (as in the example above to the English Store) can also work with PA API 5.0, please enter your other partner tags in an „Associate ID set„. All you need to do is create one set. This should be marked as default.
If ASA2 tries to send a request to the English store (as in the example), it notices that the default partner tag (of the German store) does not match the requested store. Then ASA2 checks if there is a tag for the English store in the default „Associate ID set“. If so, this one will be used and the request will work.