dmenu: 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/dmenu - from package: dmenu

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

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