public class SomeThingSomething
{
     public string Name { get; set; } = "";
    
     public Dictionary<SomeEnum, SomeDetail> SomeDict { get; set; } = new();
  
}

public class SomeDetail
{
    public string Name { get; set; } = "";
    public string Surname { get; set; } = "";
}

public class SomeThingSomethingValidator : AbstractValidator<SomeThingSomething>
{
    public SomeThingSomethingValidator()
    {
        RuleFor(md => md.Name).NotNull().NotEmpty();
       
        // Can't get this to work...
        RuleForEach(m => m.SomeDict)
    }
}

public class SomeDetailValidator : AbstractValidator<SomeDetail>
{
    public SomeDetailValidator()
    {
        RuleFor(md => md.Name).NotNull().NotEmpty();
        RuleFor(md => md.Surname).NotNull().NotEmpty();
    }
}