SYMPTOM
Azure Function trigger never fires: binding not registered
CAUSE
The Functions host does not know the trigger binding, almost always because the extension bundle is missing from host.json, or FUNCTIONS_WORKER_RUNTIME is wrong so the worker never starts.
CONFIRM IT
CONFIRM: host log on `func start` or in the portal log stream: 'The binding type(s) ... are not registered' = missing bundle. 'Did not find functions with language [x]' = wrong worker runtime. Check host.json for the extensionBundle block and app settings for FUNCTIONS_WORKER_RUNTIME.
FIX
FIX: add the extension bundle to host.json (`Microsoft.Azure.Functions.ExtensionBundle`, version `[4.*, 5.0.0)`), set FUNCTIONS_WORKER_RUNTIME to the right language, redeploy. For Service Bus/Event Hub triggers also confirm the connection app setting exists.
VERIFY
VERIFY: host log shows 'Host started' with the function listed; trigger it (HTTP call or queue message) and see the invocation log.