vimdiff: 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/vimdiff - from package: vim-enhanced

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

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