uwsgi_rack_ruby21: 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/uwsgi_rack_ruby21 - from package: uwsgi-plugin-rack-ruby2.1

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

Error in other languages:
uwsgi_rack_ruby21: bevel nie gevind nie
uwsgi_rack_ruby21: comandă negăsită
uwsgi_rack_ruby21: comando não encontrado
uwsgi_rack_ruby21: commande introuvable
uwsgi_rack_ruby21: command not found
uwsgi_rack_ruby21: käsku ei ole
uwsgi_rack_ruby21: không tìm thấy lệnh
uwsgi_rack_ruby21: komanda nerasta
uwsgi_rack_ruby21: Kommando nicht gefunden.
uwsgi_rack_ruby21: kommandot finns inte
uwsgi_rack_ruby21: komut yok
uwsgi_rack_ruby21: nie znaleziono polecenia
uwsgi_rack_ruby21: níor aimsíodh an t-ordú
uwsgi_rack_ruby21: no se encontró la orden
uwsgi_rack_ruby21: no s'ha trobat l'ordre
uwsgi_rack_ruby21: opdracht niet gevonden
uwsgi_rack_ruby21: parancs nem található
uwsgi_rack_ruby21: perintah tidak ditemukan
uwsgi_rack_ruby21: príkaz nenájdený
uwsgi_rack_ruby21: příkaz nenalezen
uwsgi_rack_ruby21: команда не найдена
uwsgi_rack_ruby21: командата не е открита
uwsgi_rack_ruby21: 命令找不到
uwsgi_rack_ruby21: コマンドが見つかりません