Seaquel
SQL Server error code

SQL Server error 3006

SQL Server raises error 3006 at severity 16, which marks a problem with the query or data that you can correct.

The differential backup is not allowed because it would be based on more than one base backup. Multi-based differential backups are not allowed in the simple recovery model, and are never allowed for partial differential backups.
Msg3006
Severity16
Group3000–3999

Source: Database Engine events and errors