sethdlc: 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/sethdlc - from package: ax25-tools
/usr/sbin/sethdlc - from package: dahdi

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

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