ht: 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/lib/picolisp/bin/ht - from package: picolisp
/usr/bin/ht - from package: tex4ht

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

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