Unfortunately, Amazon drive has been reduced to Amazon Photos in August 2019, and all third party solutions have been locked out. Some developers got a notification from Amazon, but unfortunately the Syncovery team was taken completely by surprise when it stopped working on August 22nd, 2019.
Syncovery can no longer access Amazon drive in any way. The new Syncovery 8.43 will warn users about this new situation.
We recommend choosing a more professional storage solution such as from Microsoft, Backblaze, Box, Google Drive, DropBox, or others.
If you need to download files from your Amazon drive before canceling your subscription, we recommend using the Amazon Photos app. It seems to work better than Cloud Sync on a Synology NAS. Most importantly, the Photos App will be able to show the total file count and size relatively soon after starting the sync, and you will be able to guess how long the whole download will take. As opposed to Synology Cloud Sync, which will just run for days and you never know when it might complete.