ddcmon: 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/ddcmon - from package: i2c-tools

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

On Debian possible default full paths are:

/usr/bin/ddcmon - from package: i2c-tools

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

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