Two dependencies must be enabled.
Google Backup Transport
Google Account services must be active and synced between devices.
And Google Drive app must be active (not disabled).
On a Samsung device, I was able to fix the grayed out account, I was able to restore backup functionality... but, not without the Google Drive app in an active state.
Google servers may not immediately poll a phone for backup actions immediately.
You may have to wait for piecemeal updates for a variety of data types.