MediatR - Can't create Responder [Answered]

Hi all, I'm pretty sure this is an issue with registering services, but for some reason MediatR says it can't create a responder.
Here's how I register the services - I have the main runtime and two class libraries (the plugins)
services.AddMediatR(typeof(Program), typeof(ReportBuilderPlugin), typeof(DataPlugin));

All of my responders inherit from common base class which implements IRequestHandler<DownloadDataRequest<TResponse>, Result<TResponse>>
This base responder provides some basic logging and generic functionality which is extended by some abstract methods in the implementers. Nothing I haven't done before.

Thoughts of where this might be going wrong?
I have refactored to remove the mediatr dependency.
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