If the Android 6.0 factory image isn't installed at your first attempt, then you should try to repeat the whole guide once more. (android.gs)
In case the installation process fails at your first attempt at installing it, then you can only try to repeat the entire installation once more. (android.gs)
Read this entire post and you should be able to complete the installation at your first attempt without losing any important data in the process. (android.gs)