linkchecker-gui: command not found

Have You tried accessing the binary with the full path?

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-gui - from package: linkchecker-gui

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