It is not supported version.
12.1 RU4 MP1 is first supported version.
https://www-secure.symantec.com/connect/blogs/latest-symantec-endpoint-protection-release-sep-12-ru4-mp1-12141004126
--Edit--
For install the package on Window 8.1 client you can download the latest version 12.1 RU4 MP1b client package from file connect.
https://symantec.flexnetoperations.com/control/symc/registeranonymouslicensetoken
Available client package on file connect, you can download 64 bit and install, it will be installed as unmanage.
If you want to manage it from SEPM then replace the sylink
Symantec_Endpoint_Protection_12.1.4_MP1b_Win32-bit_Client_EN.exe
Symantec_Endpoint_Protection_12.1.4_MP1b_Win64-bit_Client_EN.exe