Load Balancer shows “Status : Down” despite the droplet can be accessed by IP address.

Load balancer setting:
Forwarding rules :
HTTP on port 80 => HTTP on port 80
HTTPS on port 443 => HTTPS on port 443
TCP on port 8443 => HTTP on port 8443
(I am using 8443 port for socket.io/node.js for chat feature)

Algoristhm: Round Robin

Health checks : http://0.0.0.0:80/
Sticky sessions : Off
SSL : No redirect
Proxy Protocol : Enabled
Backend Keepalive : Disabled

Droplet Setting:
Ubuntu
The Firewall is disabled.
*I can access to the server by IP address.

I use the Cloudflare WAF.
I pointed the domain to the IP address of Load balancer.

The result is as follows..
“503 Service Unavailable
No server is available to handle this request.”

I can directly access to the droplet by the IP address.
Can anyone help me to resolve this issue ?