Trying to make a race that once per lifetime when failing death saves will revive with half HP, I can’t figure out how to make it work mechanically as a racial trait, any advice?

Are looking to make it work on dndbeyond? if so, I'm pretty sure It's impossible but I'd contact @Iamspotsa 

If your looking for balance suggestions, this is both useless and overpowered, at the same time, I'd make into lightly improved relentless endurance.

I'm thinking that it's like if you receive fatal damage, then you would remain at 1 health or something like that?

Half-Orcs and some other species get

Relentless Endurance.

When you are reduced to 0 hit points but not killed outright, you can drop to 1 hit point instead. You can't use this feature again until you finish a long rest.

This is a better feature than a once per lifetime effect.  That is frankly, not worth much, as all NPCs have to decide if they have used it or not. Having it available means the creature is a MUCH higher CR.

This sounds like a cool quirk of a racial trait - not one to hang any weight on at all, but a good theme.

I'm not one for using DDB's homebrew tools, I prefer writing it out longhand, but I disagree with people saying that it's a bad trait. Though, it could be adapted to make it more useful than once, ever...

Perhaps call it "Not Today":

Not Today: When you fail a death saving throw, and doing so does not cause you to die, then you can choose to put it aside instead of marking it as a failure. If you do this, and then stabilise, then the next time you're called upon to make death saving throws you must use the failed results you have set aside instead of rolling - these cannot be set aside again.

So you can push death back, but doing so makes it more likely that you die next time you die - but if you survive it next time, then you can use the feature again!

This is actually a significant improvement, thank you. The main reason it was once per lifetime is because the idea is that the race has two hearts and the trait kills one of the hearts, and there was going to be a magic item that would restore it, but this works way better mechanically than using an item

I'm on the side of a once-per-lifetime resurrection being a really neat idea for certain characters.  It's not anything to hang an entire species/character on, no...but man.  Imagine how awesome that cinematic moment could be, in game.  Down, dealt with, and dead..****y for their turn in initiative to come up and say four little words:

Quote from Yurei1453 >>

I'm on the side of a once-per-lifetime resurrection being a really neat idea for certain characters.  It's not anything to hang an entire species/character on, no...but man.  Imagine how awesome that cinematic moment could be, in game.  Down, dealt with, and dead..****y for their turn in initiative to come up and say four little words:

Now there's an interesting houserule - you can choose to do this instead of death saves - rise with temporary hitpoints and as an undead, but die at the next long or short rest. The "I Am Not Done!" rule, where you can be a hero and save your friends, just one last time.