vncpasswd: 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/vncpasswd - from package: tigervnc-server

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

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