virt-manager: 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-manager - from package: virt-manager

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

On Debian possible default full paths are:

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

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

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