What do I do? Someone please help me.

Is your base race marked as having subraces, using the checkbox Will Have Subraces at the bottom?

Yes. And its really annoying because its for a Warforged subrace that I think could have a lot of potential.

These are the steps that worked for me:

Click Creations -> Create Race and then Create Subrace

Then select Warforged and Create from Scratch or based on an existing one

Fill in the name and description (they can be changed later) and click Create

Now you can add traits

Yeah I did that but, but the add traits button doesn't appear. Plus, I'm doing this on a phone.

There is an issue with aspect ratios on a phone, try turning the phone sideways to landscape.

I tried but it didn't work.

Unfortunately this is an issue with your device so there's little we can do to help.

You can try using a computer. If that still has problems please raise a Support Ticket here: https://dndbeyond.zendesk.com/hc/en-us/requests/new?ticket_form_id=602788

I don't have a computer.

I don't have a computer.

FYI, D&D Beyond's mobile site has always been a bit janky.

This is the behavior that I get using my phone (Moto G5 Plus, Android, Chrome Browser, 1920x1080 screen)

On the mobile site, there is no Add Racial Trait button, in either portrait or landscape, just a +/- expansion toggle

However, if I select "Request Desktop Site" in the browser options, the Add Racial Trait button is visible in either orientation.