the program is not able to find handler for MediatR query ASP.Net Core

As I guessed, the problem was the Startup.cs where u add the MediatR service. Since my Handlers were in separate assembly so we should mention that assembly name. I changed this in Startup.cs

public void ConfigureServices(IServiceCollection services) {
    services.AddMediatR(typeof(Startup));
}

To this:

public void ConfigureServices(IServiceCollection services){
    var assembly = AppDomain.CurrentDomain.Load("Data");
    services.AddMediatR(assembly);
}

Here "Data" is the name of my assembly where all Handlers are stored there.

I needed to use:

services.AddMediatR(typeof(DeletePeopleCommand).GetTypeInfo().Assembly);

You can use this

public void ConfigureServices(IServiceCollection services){
        services.AddMediatR(AppDomain.CurrentDomain.GetAssemblies());
    }

Here GetAssemblies() will load all handler classes.