chkrootkit: 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/chkrootkit - from package: chkrootkit
/usr/sbin/chkrootkit - from package: chkrootkit

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

On Debian possible default full paths are:

/usr/sbin/chkrootkit - from package: chkrootkit

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

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