silas.mpi: command not found

Maybe Your $PATH variable doesn't contain the default paths? echo $PATH should give something like:

[root@server ~]# echo $PATH
/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin

If it's empty, You may set the PATH variable with this command

export PATH="/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin:/root/bin"

On CentOS possible default full paths are:

/usr/lib64/mpich/bin/silas.mpi - from package: scalasca-mpich
/usr/lib64/openmpi/bin/silas.mpi - from package: scalasca-openmpi

You may install the required package with command yum install {package}

Error in other languages:
silas.mpi: bevel nie gevind nie
silas.mpi: comandă negăsită
silas.mpi: comando não encontrado
silas.mpi: commande introuvable
silas.mpi: command not found
silas.mpi: käsku ei ole
silas.mpi: không tìm thấy lệnh
silas.mpi: komanda nerasta
silas.mpi: Kommando nicht gefunden.
silas.mpi: kommandot finns inte
silas.mpi: komut yok
silas.mpi: nie znaleziono polecenia
silas.mpi: níor aimsíodh an t-ordú
silas.mpi: no se encontró la orden
silas.mpi: no s'ha trobat l'ordre
silas.mpi: opdracht niet gevonden
silas.mpi: parancs nem található
silas.mpi: perintah tidak ditemukan
silas.mpi: príkaz nenájdený
silas.mpi: příkaz nenalezen
silas.mpi: команда не найдена
silas.mpi: командата не е открита
silas.mpi: 命令找不到
silas.mpi: コマンドが見つかりません