htpasswd: 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/htpasswd - from package: httpd-tools

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

On Debian possible default full paths are:

/usr/bin/htpasswd - from package: apache2-utils

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

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