I own both the DMG, Xanathars, and the PHB. When making a death domain cleric, Ray of Sickness doesn't show up, and neither does the Reaper feature of the class. There is no option to sync, as it says it's already synced and everything should be working. I need help with this

Please post a link to your character sheet so someone can look into it for you.

https://www.dndbeyond.com/characters/108754478, or www.dndbeyond.com/sheet-pdfs/MrTivoty_108754478.pdf. I'm not sure if this is how? If this doesn't work please let me know how to properly link it.

You'll see I've added some custom feats to replace the issues, like Grant Ray of Sickness, Grant Toll the Dead, and Reaper (Bugfix).

Quote from MrTivoty >>

https://www.dndbeyond.com/characters/108754478, or www.dndbeyond.com/sheet-pdfs/MrTivoty_108754478.pdf. I'm not sure if this is how? If this doesn't work please let me know how to properly link it.

You'll see I've added some custom feats to replace the issues, like Grant Ray of Sickness, Grant Toll the Dead, and Reaper (Bugfix).

 
It’s the first link, but you gotta set the character to public so we can see it.

I'm pretty sure you have a homebrew version of the Death Domain assigned to your character, and that it's put together wrong, which is why the subclass feature isn't working. On the features tab, all features should list what book they come from, with abbreviations like PHB or DMG, etc. If it's blank, like what I see for 'Death Domain Spells,' that usually means it's homebrew.

Quote from pocketmouse >>

I'm pretty sure you have a homebrew version of the Death Domain assigned to your character, and that it's put together wrong, which is why the subclass feature isn't working. On the features tab, all features should list what book they come from, with abbreviations like PHB or DMG, etc. If it's blank, like what I see for 'Death Domain Spells,' that usually means it's homebrew.

 
I kinda figured the same thing, which was why I asked for the view of the character sheet to confirm it.

Sorry. Busy at work. The character sheet should be public now. (https://www.dndbeyond.com/characters/108754478).

The abilities and features all say they’re from the PHB or DMG. I could be misunderstanding though.

You see how next to Hit Points, Proficiencies, Spellcasting, and Divine Domain it says “• PHB, pg. ##,” but next to Death Domain Spells there’s nothing? That should say “• DMG, pg. ##” there, but it doesn’t. That’s how you can tell it’s a homebrewed version of the subclass, and apparently an improperly programmed version at that.

Okay cool, I was misunderstanding. 
I’ll just make the character from scratch and use the right one, thank you so much! 

You don’t have to remake the character, just switch the subclass to the official version.