unicorn: 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 Debian possible default full paths are:

/usr/bin/unicorn - from package: unicorn

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

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