lsof: 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/sbin/lsof - from package: lsof

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

On Debian possible default full paths are:

/usr/bin/lsof - from package: lsof

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

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