Backup Exec 2012 installation error

Backup Exec 2012 installation error:
Hello,
Recently I installed the Backup Exec 2012 software on my Windows OS but it displayed an error message “Failed”. I upgraded this software to the new version and installed again but showed the same error.
I extremely need your help to fix this issue immediately.
Thanks.

Backup Exec Environment Checks
Backup Exec Version
Failed
Symantec Backup Exec version 11.0 is installed on this computer.
A version of Backup Exec later than or equal to 12.5 is required to upgrade directly to Backup Exec 2012.
