This error occurs when deploying, and causes the deployment to fail.

The DEFAULT_SCHEMA clause cannot be used with a Windows group or with principals mapped to certificates or asymmetric keys.

This is because SQL Compare and SQL Packager cannot reliably determine whether a Windows user is an actual user or a group. Windows groups can be mapped to SQL Server users just as Windows users, but a default schema cannot be specified for a Windows group. Unfortunately, not specifying a default schema for a Windows user can lead to other problems.

To fix this, note the name of the group in the error dialog, and remove that SQL Server user from the deployment.