Yousef,
As per Symantec DLP Installation Guide, the import of a solution pack must be performed right after the initial installation, without any Detection Server installed.
There can be no Detection Servers, policies, additional users or roles added. The Solution Pack must be installed before anything else.
If you wish to install a Solution Pack after the database has been used, it must be cleared. Reinstall the Enforce Server and select the Initialize Enforce Data option. This will recreate the database, removing anything previously added, including incidents.