fvwm-crystal: command not found

Have You tried accessing the binary with the full path?

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/fvwm-crystal - from package: fvwm-crystal

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