Hi @TI HPF ,
Thank you for your reaching out and welcome to Microsoft Q&A!
The SQL Server database files (*.mdf, *.ndf and *.ldf) and backups are not backward compatible. Backward compatibility is why we cannot restore or attach a database created from a higher version of SQL Server to a lower version of SQL Server. However, there are a few options that can help us to downgrade the database from a higher version of SQL Server to a lower version SQL Server.
You could refer to How to migrate a SQL Server database to a lower version if you want to restore the backup from a newer version of SQL Server to an older.
Best regards,
Lucy Chen
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