mpitests-osu_reduce_scatter: 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/mpitests-osu_reduce_scatter - from package: mpitests-mpich
/usr/lib64/mvapich2-psm/bin/mpitests-osu_reduce_scatter - from package: mpitests-mvapich2-psm
/usr/lib64/mvapich2/bin/mpitests-osu_reduce_scatter - from package: mpitests-mvapich2
/usr/lib64/mvapich-psm/bin/mpitests-osu_reduce_scatter - from package: mpitests-mvapich-psm
/usr/lib64/mvapich/bin/mpitests-osu_reduce_scatter - from package: mpitests-mvapich
/usr/lib64/openmpi/bin/mpitests-osu_reduce_scatter - from package: mpitests-openmpi

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

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