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 CentOS possible default full paths are:

/usr/bin/top - from package: procps

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

On Debian possible default full paths are:

/usr/bin/top - from package: procps

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

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