how can i resolve 400 bad request The plain HTTP request was sent to HTTPS port issue on nginx server

Is your NLB doing the SSL offloading? So HTTPS is coming in the front on port 443, the NLB decrypts the traffic, and sends plain HTTP out of the back https://aws.amazon.com/blogs/aws/new-tls-termination-for-network-load-balancers/

If this is what's happening then this needs to be directed to the plain HTTP port 80 on the EC2, not to port 443 which is expecting HTTPS.

Probably not related, but you've got a single server process listening for HTTP & HTTPS and while this is legal and should work as long as the syntax is correct, consider breaking it into two separate servers:

server {
    listen 80;
    server_name myserver.example.com;
    return 301 https://myserver.example.com$request_uri;
}

server {
    listen 443 ssl;
    server_name myserver.example.com;

    ssl_certificate /path/to/my/cert;
    ssl_certificate_key /path/to/my/key;

    [ any other config ]
}

The error indicates the outbound communication from the NLB doesn't use TLS.

To send traffic using TLS from the NLB to your target group you must set the target group protocol to TLS https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-target-groups.html#target-type:~:text=a%20target%20group-,Routing%20configuration,-By%20default%2C%20a

If a target group is configured with the TLS protocol, the load balancer establishes TLS connections with the targets using certificates that you install on the targets.

You also have the option of creating an NLB that passes through encrypted traffic without decrypting it by creating a TCP listener and setting the port to 443. See https://docs.aws.amazon.com/elasticloadbalancing/latest/network/load-balancer-listeners.html#:~:text=Network%20Load%20Balancer-,Listener%20configuration,-Listeners%20support%20the

With a TCP listener, the load balancer passes encrypted traffic through to the targets without decrypting it