A cloud-based identity and access management service for securing user authentication and resource access
It is most likely due to an invalid attribute on the synced account.
You can open synchronization service on the entra connect server and look for errors which will show in the pane.
Also see to fix invalid attributes: https://learn.microsoft.com/en-us/microsoft-365/troubleshoot/active-directory/duplicate-attributes-prevent-dirsync?source=recommendations
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola