expr: 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/bin/expr - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/expr - from package: coreutils

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

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