Hello.  I'm currently working on a home-brew Aarakocra variant and I'm trying to edit its size through the add racial trait section.  I am following the instructions from the following guide, under the variant section:
https://www.dndbeyond.com/posts/1182-how-to-create-a-homebrew-race-using-d-d-beyond

I go to add racial trait, I fill in what the guide tells me, give it a name (Size), fill in snippet and description (You are Small.), I choose a display order (4), and I click save.  Every time I do that, the page just reloads and nothing is saved.

I already have a racial trait I wrote on my own in the home-brew race, so I can add things.

Does anyone have any idea what is happening and how I can fix it?

You are forgetting to designate it as a “granted” trait.

Thanks.  The guide didn't mention anything about it that I can see, so I didn't know it had to be done.  

Yup, all features & traits must be designated as either “Granted,” “Additional,” or “Replacement.” But you will only ever want anything other than “Granted” if you want to set up optional class features or origins customizations like from Tasha’s Cauldron, so just always choose “Granted.”