To perform the requested action, WordPress needs to access your web server. Please enter your FTP credentials to proceed.
Under that are the text boxes for Hostname, FTP User, and FTP Password, and then radio buttons for FTP or FTP (SSL).
Solution: It is a permissions problems and can be easily fixed. If your webserver is running with nginx as user and nginx as group, then try the following command on the directory serving the web pages:
chown -R nginx:nginx /var/www