Hi @Bill Fry
Thank you for asking this question on the **Microsoft Q&A Platform. **
This thread may help:
The error Dn-Attribute-failure usually occurs when there are duplicate attribute values exist for 2 or more users/groups/contacts in the on-premises domain but those values cannot be assigned to multiple users in Azure AD. For example, you can have the same SMTP/Proxy address configured for 2 users in local AD, but when you sync those users to Azure AD, you will encounter a Dn-Attribute-failure error as the value of these attributes must be unique for every user in Azure AD.
Hope this helps,
Carlos Solís Salazar
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