我的法律文本遇到了同样的问题。

I encountered the same issue with the legal text.

按照 DOC ：

请注意：为了保护用户隐私，该设备用户必须选择加入到   谷歌服务的自动备份工作。在谷歌服务的选择，在   当用户经过安装向导对话框出现，或   配置设备上的第一个谷歌帐户。

Note: To protect user privacy, the device user must have opted in to Google services for Auto Backup to work. The Google services opt-in dialog appears when the user goes through the Setup Wizard or configures the first Google account on the device.

但问题是，我从来没有看到这个对话框。 所以我清除了谷歌播放服务的所有数据，禁用它，再次启用它，打开Play商店应用。我不得不添加我的帐户，并在过程结束时，对话框出现了。

The problem was that I never saw this dialog. So I cleared all the data of Google play services, disabled it, enabled it again and opened the Play Store app. I had to add my account and at the end of the process, the dialog appeared.

那么，不要忘记更新谷歌播放服务的最后一个版本，否则你不会有分配备份帐户。在更新结束后，你应该能够执行命令亚行外壳BMGR fullbackup＆LT;包装＆GT; 正确

Then, don't forget to update Google Play Services to the last version else you won't have a backup account assigned. At the end of the update, you should be able to execute the command adb shell bmgr fullbackup <PACKAGE> properly.