linkchecker: 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/linkchecker - from package: linkchecker

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

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