acpi: 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/acpi - from package: acpi

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

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