pg_top: 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/pg_top - from package: pgtop

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

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