You are being too specific in the signature of my_sum, requiring that the argument be a Vector{Float64}. It ends up being passed a SubArray and there is no matching method.
In most cases you don’t need to be so specific about argument types when you define a method. In fact, it is often an advantage not to assign types to arguments if there is only one method. Or you could give it a more general type like AbstractVector{<:AbstractFloat}