nano: 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:

/bin/nano - from package: nano
/usr/bin/nano - from package: nano

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

On Debian possible default full paths are:

/bin/nano - from package: nano
/usr/bin/nano - from package: nano

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

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