You want to edit a list using the datasheet view (now called Quick Edit), but the Quick Edit option is greyed out. When you hover over it, a message appears:
This control is currently disabled.Problem:
In my case, it was because the list had a non-default style applied to the view. It knocks out the datasheet editing.
I modified the view and changed the style to default. Clicked on ok, then the Quick Edit was once again available.
Hopefully, Microsoft will address this. It seems like a terrible bug.