compare-im6: 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/compare-im6 - from package: ImageMagick

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

Error in other languages:
compare-im6: bevel nie gevind nie
compare-im6: comandă negăsită
compare-im6: comando não encontrado
compare-im6: commande introuvable
compare-im6: command not found
compare-im6: käsku ei ole
compare-im6: không tìm thấy lệnh
compare-im6: komanda nerasta
compare-im6: Kommando nicht gefunden.
compare-im6: kommandot finns inte
compare-im6: komut yok
compare-im6: nie znaleziono polecenia
compare-im6: níor aimsíodh an t-ordú
compare-im6: no se encontró la orden
compare-im6: no s'ha trobat l'ordre
compare-im6: opdracht niet gevonden
compare-im6: parancs nem található
compare-im6: perintah tidak ditemukan
compare-im6: príkaz nenájdený
compare-im6: příkaz nenalezen
compare-im6: команда не найдена
compare-im6: командата не е открита
compare-im6: 命令找不到
compare-im6: コマンドが見つかりません