For patching and installation troubleshooting, refer to the general troubleshooting guide.
Navigate to your device settings > Accounts > Manage accounts > Google (blue icon) > Remove account. You can sign in again if you want.
If that doesn't work you can uninstall GmsCore and then reinstall GmsCore.
To understand and set up the download feature, refer to section 2 of the setup info guide.
Refer to section 3 of the setup guide.
Google has made it so that Canadian users who do not have YT Premium can only use the Radio feature within YouTube Music. To solve this, refer to section 3 of the setup guide.