dnstop: 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/dnstop - from package: dnstop

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

On Debian possible default full paths are:

/usr/bin/dnstop - from package: dnstop

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

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