yi: 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/yi - from package: yi

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

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