portreserve: 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:

/sbin/portreserve - from package: portreserve

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

On Debian possible default full paths are:

/sbin/portreserve - from package: portreserve

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

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