When choosing to edit a list in Microsoft SharePoint Datasheet View the list or view is read-only. You can observe this by seeing the message "This view is read-only." at the bottom of the datasheet.
The Microsoft SharePoint Content Approval setting has been turned on for the list. The Datasheet View control does not support making edits to lists that have Content Approval turned on.
Make your edits in Microsoft SharePoint Standard View or turn off Content Approval.
To turn off Content Approval in Microsoft SharePoint do the following:
In the List select Settings
Select List Settings
Select Versioning Settings
In the Content Approval section select No for "Require content approval for submitted items"