This error means, that the file, you try to upload is bigger than the configured max file size.
This is done to prevent users to fill up the space on your server with enormous files.

You can change this value in the configuration file.

[tux@server]$ vim /etc/nginx/nginx.conf

...  
http {
	...  
	client_max_body_size 0;
	...  
}
...  

If this line is not present in your configuration, you can simply add it.
A value of 0 disables this feature completely, which may be risky if you users have the possibility to upload files. If only you as the administrator has the ability it is fine.

For the changes to take effect, you need to reload the configuration of nginx.

[tux@server]$ systemctl reload nginx