icalmerge: 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/icalmerge - from package: ical2html

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

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