Umbraco Models Builder throwing error on Run Custom Tool

I am trying to work with Umbraco Models Builder with the following settings in web.config -

 <add key="Umbraco.ModelsBuilder.Enable" value="true" />
 <add key="Umbraco.ModelsBuilder.EnableApi" value="true" />

I have installed ModelsBuilder.Api nuget pakage as well.

I am generating the models in a seperate project and when I run custom tool I get the following error -

UmbracoModelsBuilder: UmbracoModelsBuilder failed to generate code: Exception: Response status code does not indicate success (Unauthorized)

I have added the credentials in ModelBuilder options as well (Tools-> Options-> Umbraco-> ModelsBuilder options).

Can someone help me with this.

Thanks