rkhunter: 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/rkhunter - from package: rkhunter

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

On Debian possible default full paths are:

/usr/bin/rkhunter - from package: rkhunter

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

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