Amazon: Purchasing failed to initialize. Reason: NoProductsAvailable

Unity IAP requires two components, the asset package (1.23.1 for example) and the corresponding In App Purchasing library in Package Manager (2.0.6). The latest combination is 2.2.5/2.2.1. So you need to check both. You can view the asset version from menu Window/Unity IAP/IAP Updates… You can install previous versions from here. Make sure to make a full project backup before changing any versions Unity IAP Previous Versions (Obsolete - please see Package Manager)