Unable to create the folder (Nginx config)

Mots likely you used a wrong php-fpm port. Ecah website has its own php-fpm instance which uses its own port. If you use a different port, then php runs under a wriong user and you cant use functions that need to store data on the harddisk in php.

try to remove this complete part:

location ~ \.php$ {
try_files $uri =404;
include /etc/nginx/fastcgi_params;
fastcgi_pass 127.0.0.1:8999;
fastcgi_param SCRIPT_FILENAME $document_root$fastcgi_script_name;
}

as the php handlr is set by ispconfig automatically.