ok first things first, the SMSTSPostAction variable is for use in operating system deployment task sequences to do an action after the task sequence has completed,
are you planning on installing SEP as part of a task sequence ? if so use an Install Application step instead of Install Package if that's easier, or even run PowerShell script...
but first, if you really want to install the app just using a powershell script then test the script on a virtual machine standalone, outside of a task sequence