What to do when I can't download iCloud backup file?

1. Check your internet connection is OK.

Please make sure your internet can connect to iCloud server, you can visit https://www.icloud.com/ to check.
Also, you can try to download iCloud backup file under another internet connection.

2. Make sure you have disabled Two-Step verification in your iCloud account.

Two-Step verification will stop Toolbox from downloading iCloud backup file, so please go to https://appleid.apple.com/#!&page=signin to disable Two-Step verification.

3. You can now run Toolbox again to download iCloud backup file.