I have added a field in my attribute table but I cannot calculate the area. The units field is greyed out and the options at the top say "area - disabled".
What can I do about this?
I think your feature layer is not projected into Projected Coordinate System(e.g UTM). So re-project Geographic Coordinate Systems (e.g WGS84) into Projected (e.g UTM) and then you will be able to calculate area.