yacpi: 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/yacpi - from package: yacpi

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

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