The output indicates you have 2 MPI processes in your application. The ranks of those processes in MPI_COMM_WORLD would be 0 and 1, so 2 is not a valid destination rank for your message. If your application requires more than 2 processes, you'll need to amend your launch settings to start more. Ken On 5/18/21, 2:47 PM, "Parviz Ghahramani via discuss" <[email protected]> wrote: Hi, I am trying to run a process for parallelization over 4 cores. This is run on Windows EC2 instance on AWS. Initially I was getting error message that MPI rejected credentials. I entered and registered the credentials. Now I get the following message “Invalid rank”. USING PARALLEL PROFILE C:\nm75g64\run\para300.pnm MPI TRANSFER TYPE SELECTED Building NONMEM Executable Starting MPI version of nonmem execution ... Manager Location EC2AMAZ-EP7B8E1\\c:\nm75g64\run Fatal error in MPI_Send: Invalid rank, error stack: MPI_Send(173): MPI_Send(buf=00000000037FFD40, count=1, MPI_INTEGER, dest=2, tag=2, MPI_COMM_WORLD) failed MPI_Send(98).: Invalid rank has value 2 but must be nonnegative and less than 2 job aborted: rank: node: exit code[: error message] 0: localhost: 1: process 0 exited without calling finalize 1: localhost: 123 Can anyone provide guidance what this message is and how I can resolve it? Kind regards Parviz