Add User button greyed out for custom role

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Dear Oracle Connects,

I have created one custom role in which I only have to give priviledge of View Purchase Order & Search Purchase Order. After adding priviledge, I am unable to add user to the role as Add User button is greyed out. I have referred the below Oracle SR note.

Add Role button is greyed out in Security Console (Doc ID 2343822.1)

Here it is mentioned to select  Enable edit of user role membership' option from Administration

I am unable to see this option under Role in Administration tab. What can be probable reason for this? How can I assign this custom role to the User?