vncviewer: 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/vncviewer - from package: tigervnc

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

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