The issue is with the phases that the Elixir compiler goes through.

While this

defmodule Subscriber do
  defstruct name: "", paid: false, over_18: true

end
s1 = %Subscriber{}
IO.puts inspect(s1)

results in an error, the following does not

defmodule Subscriber do
  defstruct name: "", paid: false, over_18: true

  def new,
    do:  %Subscriber{}

end
s1 = Subscriber.new()
IO.puts inspect(s1) 

During compilation the expansion phase happens before the evaluation phase. However the functionality of a module isn’t “defined” and available until it has been evaluated during the evaluation phase. So when the compiler comes across

s1 = %Subscriber{}

during the expansion phase it tries to expand %Subscriber{} - the problem is that %Subscriber{} is part of the Subscriber module which won’t be defined until later in the evaluation phase - so during the expansion phase there is no definition for %Subscriber{} (yet).

s1 = Subscriber.new()

doesn’t have anything to expand - it is simply a function call that is evaluated during the evaluation phase - by that point in time the Subscriber module has been fully evaluated and both %Subscriber{} and Subscriber.new() are defined and available.

See also

Main phases of Elixir compilation