When trying to add a Spell to a Homebrew feature for the Paladin Subclass, the Find Greater Spell is missing.

All other spells are available, just Find Greater Steed is missing.

 Hello WitheringAurora! Find greater steed is from Xanathar's Guide to Everything. You would need to own either that title, or the spell individually in order to use it in your homebrew creations.

I see, I assumed I'd be able to access it seeing as when I try adding Mordekainen's Private Sanctum to the list of spells it works perfectly. 

But Find Greater Steed is in fact missing.

Quote from WitheringAurora >>

I see, I assumed I'd be able to access it seeing as when I try adding Mordekainen's Private Sanctum to the list of spells it works perfectly. 

But Find Greater Steed is in fact missing.

 
Mordenkainen's Private Sanctum is from the Player's Handbook (PHB).

Find Greater Steed is from Xanathar's Guide to Everything (XGtE).

Seems you have access to the PHB but not XGtE. 

You can buy the XGtE book or just get the individual spell from the Marketplace.

Apoligies, I read the PhB part wrong on that spell.

But I do have access to other XGE spells.

such as Earth Tremor, Earth Bind, Erupting Earth and the like.

Quote from WitheringAurora >>

Apoligies, I read the PhB part wrong on that spell.

But I do have access to other XGE spells.

such as Earth Tremor, Earth Bind, Erupting Earth and the like.

 Those three spells are available for free as a part of the Elemental Evil Player's Companion. You can check your purchases in your Order History. :)