Unable to make Attribute as Key Attribute

Hi All,

I am trying to mark an attribute as Key Attribute in Attribute view.

Tried with below options.

a. Attribute View -> Schema -> Type but not able to mark it as Key Attribute

b. Attribute View -> Schema -> Select the Attribute , Right Click and Add it as Key Attribute

c. Tried to change with the properties (marked in blue), but in vain.

Are there any pre-requisites?

Regards,

Rebecca