wkhtmltopdf: 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 Debian possible default full paths are:

/usr/bin/wkhtmltopdf - from package: wkhtmltopdf

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

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