BMXAA7107E - site is not allowed because the Site of the classification must match the Site of the children and attributes.

 Go to selection action associated attributes 

Their site id is mentioned on your attribute i.e why when u are crating attriute without siteid its not taking as you need to remove that attribute from associated list.

another reason could be classification or child classification in associate classification dialog has siteid and orgid i.e it its giving this error.

check index on field or checking index before deliting in sql explorer oracle

 Right click on connection.

Select Schema browser

Select your schema

Select you table 

Go to Index tab

you will be able to see unique indexes.