htop: 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 CentOS possible default full paths are:

/usr/bin/htop - from package: htop

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

On Debian possible default full paths are:

/usr/bin/htop - from package: htop

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