Unable to Use Generate Sample Data (AI Copilot)

Summary:
The Generate Sample Data feature for a Record Type fails. The issue also reproduces with a newly created, minimal Record Type.

Environment: News / Region: Unknown / Appian Version: 25.4

Issue:
In Data Preview → Generate Sample Data, the following message appears:

We encountered an issue and could not generate your sample data.

Click REFRESH ALL DATA in the Additional Configuration panel and try again

What we’ve verified:

• Database permissions and data sync are normal.
• Source = Database / Sync = Enabled / Primary Key = Integer (also fails for a newly created Record Type).
• AI services available: Claude Sonnet 4.5 and Claude Haiku 4.5.
• The same issue reproduces for multiple users in the same environment.