Hello,
Symantec_Endpoint_Protection_12.1.2_MP1_Win64-bit_Client_EN.exe is for unmanaged SEP clients.
In your case, you may have to download the "Symantec_Endpoint_Protection_12.1.2_MP1_SEPM_EN.exe"
OR
Symantec_Endpoint_Protection_12.1.2_MP1_Part1_Installation_EN.exe
which includes the packages which you would have to import to the SEPM.
Symantec sends you client installation package updates, and then you add them to the Symantec Endpoint Protection database to distribute them from the Symantec Endpoint Protection Manager.
You can optionally export the packages during this procedure to make the package available for deployment to computers that do not contain client software.
Symantec_Endpoint_Protection_12.1_RU1_MP1_SEPM_EN\Packages
OR
Symantec_Endpoint_Protection_12.1_RU1_MP1_Part1_Installation_EN\SEPM\Packages
An installation package that you import consists of two files. One file is namedproduct_name.dat, and the other file is named product_name.info.
Adding client installation package updates
http://www.symantec.com/docs/HOWTO81175
Hope that helps!!