My goal is to make subraces for the dragonborn. I have made a homebrew copy of the dragonborn and checked the box that says "Will have subraces?" The details page of the homebrew says "This homebrew Race has been set to have subraces, but no subraces have been created."

However, when I go to the Create Subrace page, the "Select a race to create a new Subrace for" dropdown menu does not display my COPY_OF_Dragonborn as an option. I've tried renaming the race, I've tried creating a new parent race from scratch, I've tried using a race that normally has a subrace (dwarf) as a template. None of these things have shown up in the parent race dropdown for making a subrace. 

Hello! When in the edit page for your homebrew race, can you let me know what result you get when you select "Add a Subrace"?

That works! Thank you! Consider my problem resolved. 

When I do it then it still wont work

First you have to designate that the main race will have subraces like MellieDM said. Then you have to save the main race. Then you can add the subraces. Then you have to save the main race again.