virt-viewer: 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/virt-viewer - from package: virt-viewer

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

On Debian possible default full paths are:

/usr/bin/virt-viewer - from package: virt-viewer

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

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