Hi, I'm trying to create a magic item that allows the attuned player to cast Ray of Sickness but it doesn't seem to appear in the choices.  Thanks!

Ray of Sickness is from the player's handbook - you'll need access to that on D&D Beyond for it to appear as an option in the homebrewer.

That means you will either need to purchase the PHB from DDB, or at least that one spell.

I hope this was helpful.

I'm part of a campaign that does.  I've made one or two items already with spells from PHB.

Quote from Cadrig >>

I'm part of a campaign that does.  I've made one or two items already with spells from PHB.

 Being part of a campaign with content sharing does not allow you access to those materials for homebrew, the materials must be unlocked by you. The items you made before most likely used spells that also appeared in the Basic Rules & SRD, and therefore are free for everyone to use. Of the officially published spells, only these can be attached to a homebrew without purchase since these are the only ones WotC published for free:

https://www.dndbeyond.com/spells?filter-class=0&filter-search=&filter-verbal=&filter-somatic=&filter-material=&filter-concentration=&filter-ritual=&filter-sub-class=&filter-source=1&filter-source=4

That list contains every spell published as part of the Basic Rules & SRD, as well as the Elemental Evil Player’s Companion which was also published by WotC for free.

I hope this was helpful.

Edit: There is a workaround however. Whoever is in that campaign with you who owns the PHB can make that magic item, and as long as they keep it in their “collection” (even if it is not published) then everyone in the campaign could potentially use one, even you.

 Okay that makes a lot more sense.  Thank you that was very helpful!