dmenu_run: command not found

Have You tried accessing the binary with the full path?

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_run - from package: dmenu

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

On Debian possible default full paths are:

/usr/bin/dmenu_run - from package: suckless-tools

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