pr: 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/pr - from package: coreutils

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

On Debian possible default full paths are:

/usr/bin/pr - from package: coreutils

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

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