fzn-gecode: 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 CentOS possible default full paths are:

/usr/bin/fzn-gecode - from package: gecode-devel

You may install the required package with command yum install {package}

On Debian possible default full paths are:

/usr/bin/fzn-gecode - from package: flatzinc

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

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