The backend server group's protocol (backend protocol) you want to select is not supported by the listener protocol (frontend protocol). There are some constraints on the backend protocol when you associate a backend server group with a listener.

**Table 1** Frontend and backend protocols of dedicated load balancers
Frontend Protocol	Backend Protocol
TCP	TCP
UDP	UDP/QUIC
HTTP	HTTP
HTTPS	HTTP/HTTPS

**Table 2** Frontend and backend protocols of shared load balancers
Frontend Protocol	Backend Protocol
TCP	TCP
UDP	UDP
HTTP	HTTP
HTTPS	HTTP

Why Can't I Select the Target Backend Server Group When Adding or Modifying a Listener?