virt-what: 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/sbin/virt-what - from package: virt-what

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

On Debian possible default full paths are:

/usr/sbin/virt-what - from package: virt-what

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

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