I have installed Horizon. The window opens, but hangs (The loading gif in the center never goes away... site tree doesnt load...). I see in the logs:

Error in GraphQL query (parsing/system): GraphQL.ExecutionError: {"Message":
"No HTTP resource was found that matches the request URI 'https://myhost.com/sitecore/api/ssc/horizon/sites/-/getsites?name=mysite&sc_horizon=api'.",
"MessageDetail":"No type was found that matches the controller named 'sites'."}
2019-12-14T09:34:15.2313517-05:00 [ERR] (SerilogSample/PHLWLTR90V1MYD) Error processing GraphQL query
GraphQL.ExecutionError: Error trying to resolve name. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at Sitecore.Horizon.API.Gateway.Schema.GraphTypes.SiteGraphType.<>c.<.ctor>b__1_0(ResolveFieldContext`1 context)
at GraphQL.Resolvers.FuncFieldResolver`2.GraphQL.Resolvers.IFieldResolver.Resolve(ResolveFieldContext context)
at GraphQL.Execution.ExecutionStrategy.ExecuteNodeAsync(ExecutionContext context, ExecutionNode node)
--- End of inner exception stack trace ---
2019-12-14T09:34:15.2317627-05:00 [ERR] (SerilogSample/PHLWLTR90V1MYD) Error processing GraphQL query
GraphQL.ExecutionError: Error trying to resolve language. ---> 
System.NullReferenceException: Object reference not set to an instance of an object.
at Sitecore.Horizon.API.Gateway.Schema.GraphTypes.SiteGraphType.<>c.<.ctor>b__1_3(ResolveFieldContext`1 context)
at GraphQL.Resolvers.FuncFieldResolver`2.GraphQL.Resolvers.IFieldResolver.Resolve(ResolveFieldContext context)
at GraphQL.Execution.ExecutionStrategy.ExecuteNodeAsync(ExecutionContext context, ExecutionNode node)
--- End of inner exception stack trace ---
2019-12-14T09:34:15.2320987-05:00 [ERR] (SerilogSample/PHLWLTR90V1MYD) Error processing GraphQL query
GraphQL.ExecutionError: Error trying to resolve startItem. ---> System.NullReferenceException: Object reference not set to an instance of an object.
at Sitecore.Horizon.API.Gateway.Schema.GraphTypes.SiteGraphType.ResolveStartItem(ResolveFieldContext`1 context)
at GraphQL.Execution.ExecutionStrategy.ExecuteNodeAsync(ExecutionContext context, ExecutionNode node)
--- End of inner exception stack trace ---

I have looked at ShowServicesConfig.aspx and I see that the Horizon SitesController is registered, so I am inclined to think its a routing problem? But I am unsure.