If you are trying to upload large files and are seeing this error in the nginx error log, it means the client_max_body_size value is too low.
Open the nginx configuration file:
$ sudo nano /etc/nginx/nginx.conf
It may also be located in /usr/local/nginx/conf/nginx.conf or /usr/local/etc/nginx/nginx.conf
Add the following line within the http block
http {
. . .
client_max_body_size 64M;
. . .
}
Save and exit (press CTRL + X, press Y and then press ENTER)
Now, check if the Nginx config file syntax is valid.
$ sudo nginx -t
If valid, restart Nginx service.
$ service nginx restart
You may also need to edit the php.ini configuration file.