When I search for products, I receive the error message "Your [AWS access key] is not registered as an Amazon Associate. Please register as an associate at https://affiliate-program.amazon..."

This means that your Amazon account is not activated for the partner program of the country selected for your search. Please visit the URL mentioned in the error message and try to activate your account for that country. If that does not help, contact the Amazon support and ask them to activate your account for all countries.