This section needs to be in inside your HTTP context ie - HTTP { }
server {
listen xxx.xxx.xxx.xxx:443 ssl http2;
server_name unifi.yourdomain.com;
ssl_certificate /etc/nginx/acme.sh/yourdomain.com/fullchain.pem;
ssl_certificate_key /etc/nginx/acme.sh/yourdomain.com/key.pem;