Let's say something like this is being returned.

AH01144: No protocol handler was valid for the URL /helloworld (scheme 'balancer'). If you are using a DSO version of mod_proxy, make sure the proxy submodules are included in the configuration using LoadModule.

I typically find this suggest some issue with one of the proxy modules. The apachectl command with the -M flag can be used to display the modules that are currently loaded. Check out my article Display loaded modules using the apachectl command.

<web_server_root>/bin/apachectl -M

Which should return something like this.

Loaded Modules:
  proxy_module (shared)
  proxy_http_module (shared)
  proxy_balancer_module (shared)
  lbmethod_bybusyness_module (shared)
  lbmethod_byrequests_module (shared)
  lbmethod_bytraffic_module (shared)
  lbmethod_byheartbeat_module (shared)

The httpd.conf file will contain a line for each module, like this:

LoadModule proxy_module                modules/mod_proxy.so
LoadModule proxy_http_module           modules/mod_proxy_http.so
LoadModule proxy_balancer_module       modules/mod_proxy_balancer.so
LoadModule lbmethod_bybusyness_module  modules/mod_lbmethod_bybusyness.so
LoadModule lbmethod_byrequests_module  modules/mod_lbmethod_byrequests.so
LoadModule lbmethod_bytraffic_module   modules/mod_lbmethod_bytraffic.so
LoadModule lbmethod_byheartbeat_module modules/mod_lbmethod_byheartbeat.so

And the module .so files should be in the modules directory.

<web_server_root>/modules/mod_proxy.so
<web_server_root>/modules/mod_proxy_http.so
<web_server_root>/modules/mod_proxy_balancer.so
<web_server_root>/modules/mod_lbmethod_bybusyness.so
<web_server_root>/modules/mod_lbmethod_byrequests.so
<web_server_root>/modules/mod_lbmethod_bytraffic.so
<web_server_root>/modules/mod_lbmethod_byheartbeat.so

For more details on this, check out my article on Load Balancing Proxy.