This problem rarely occurs. It is probably because of the connection from your web server to the server where the update package is located. In most cases it worked at a later point in time, e. g. one day later.
If this still doesn’t work, you can update manually. Download the ASA2 zip-package from https://getasa2.com/your-account and exchange the plugin directory „amazon-simple-admin“ with the one from the zip-package. Important: You don’t have to disable or even delete ASA2 in the WordPress plugin area, because that would destroy your settings. Just replace the directory and check in the ASA2 dashboard if the plugin status is ok. If not, click the „Perform selftests“ button.