Data Migration : How update some existing contacts

Hello,

I would like be able to update some existing contact records, using the Account ID and the Contact ID as key in the Data Migration tool, but it doesn't.

I got this kind of message, while the Account and the Contact exist and are linked :

I use the Upload properties "Modify" and I only try to update some information of the contact (tab : "Contacts" in the Data Migration template)

Thank you for your help

Regards

Oliver