grefer: 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/grefer - from package: groff

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

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