Unique property on aras

Most probably you have Items in your database for this particular 'item type' where the property you are trying to mark as unique has duplicate values.

No actually this is a brand new property I created

Do you have some other property marked as unique on Item Type SubContract1?

Nopes there is none also I tried changing the name of the property as well but it would give the same error. 

Can you share some screenshots? Like
1. filter properties which are unique
2. Items grid of Subcontract1 to just have glimpse of data available 

Are you trying to make the property from not unique to unique ? if yes, Is there any Subcontract1 item already created in system ? If yes, then set some unique value to those data and then try making this field to unique.

Are you trying to make the property from not unique to unique ? if yes, Is there any Subcontract1 item already created in system ? If yes, then set some unique value to those data and then try making this field to unique.