fvviewer: 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/fvviewer - from package: fawkes-guis

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

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