ps: 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:

/bin/ps - from package: procps

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

On Debian possible default full paths are:

/bin/ps - from package: procps

You may install the required package with command apt-get install {package}

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