dmenu.default: 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 Debian possible default full paths are:

/usr/bin/dmenu.default - from package: suckless-tools

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

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