Datasphere : Why we cannot replicate data in delta from CO-PA Datasource ?

[#1]

I try to create a replication flow from the standard ECC CO-PA datasource to Datasphere, I cannot set the delta mode because of this message :

"Replicating ABAP-based objects that do not have a primary key is only possible for load type "Initial Only".

Any idea ? I suppose the CO-PA table did have a primary key !

Thanks.

Vincent.