cu: 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/cu - from package: uucp

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

On Debian possible default full paths are:

/usr/bin/cu - from package: cu

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

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