At the end of the index rebuild process it will switch the 2 Aliases over the 2 collections.
To put the rebuilt index live, and the old index under the rebuild alias.
Do your two collections exist in Solr? In particular "sitecore_custom_web_index".
If your collection doesn't exist it will error trying to set an Alias for it.
You can debug the commands getting sent to Solr, by running your iis app pool as your current user, changing the solr connection string from localhost:port to localhost.:port (notice the .) and running fiddler.
Or looking in the Solr logs.
Or construct it from documentation and your configuration.
Once you know the Solr command, you can try calling Solr with that command to set the Alias yourself and see if works/fails.
Another note, the collection api only works in SolrCloud mode, you mention you have Zookeeper so should be fine. Check that in Solr you can see the Cloud left hand nav link, and the graph page shows a healthy green cluster.