varnishlog: 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/varnishlog - from package: varnish

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

On Debian possible default full paths are:

/usr/bin/varnishlog - from package: varnish

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

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