smidiff: 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/smidiff - from package: libsmi

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

On Debian possible default full paths are:

/usr/bin/smidiff - from package: smitools

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

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