mkfifo: 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/mkfifo - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/mkfifo - from package: coreutils
/usr/lib/klibc/bin/mkfifo - from package: klibc-utils

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

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