fconvolver: 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/fconvolver - from package: jconvolver

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

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