Hi there. I'm the DM just in case that matters.

I'm using Beyond and adding a custom race + subraces. Each subrace gets a cantrip (even non-spell casting classes). But it's not adding it. It's not ready for even close to public publication, but I'm still struggling. It's appearing on a member of the subrace who already has magic, but it's not being added.

I don't know how to Google to get help to fix it and I'm just frustrated I can't override the spell section.

You most likely forgot to designate a spellcasting ability, so the system is only showing the spells for spellcasters because of the Spellcasting ability granted by their class. Please see the Homebrew Subclasses & Races FAQ #14 for more information: (https://www.dndbeyond.com/forums/dungeons-dragons-discussion/homebrew-house-rules/131411-a-homebrewers-how-to-faq). It was written for Subclasses, but it’s pretty similar for Races.

I guess as a correction, the character is a spell caster but the cantrip won't show up. I've just found a feat to deal with it until I can get it to work. 

Follow that link I provided and follow the instructions in the indicated answer and it’ll work. You might also need to check out the General FAQ #6 at the same link.