Unlike IC5141, IC61x versions does not have simulators bundled in it. All simulators (Spectre, Spectre RF, UltrasSim, APS) are now part of MMSIM package. You need to have MMSIM installed if you are using IC61x version.
As Quek mentioned, type “spectre –W” to get the exact version of Spectre that you are using. If you don’t have MMSIM installed and don’t have it in your path, then you would get “command not found”.