webmail-install: 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/webmail-install - from package: php-horde-webmail

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

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