I don't see Necklace of Adaptation in my infusions list to add for my level 10 options 

is there a reason its not showing up?

I can confirm that the infusion Replicate Magic Item - Necklace of Adaptation doesn't show up for me either when I level my Artificer to 10 or higher.

I can also confirm this.

I would check this for myself, but due to DDB automatically pushing updates on us and not even letting us see the older versions anymore, I cannot. If anyone has a hardcopy of Eberron: Rising, could you check to see if it was omitted from the original list and then subsequently added with Tasha’s? DDB might have just missed it if that’s the case.

Quote from IamSposta >>

I would check this for myself, but due to DDB automatically pushing updates on us and not even letting us see the older versions anymore, I cannot. If anyone has a hardcopy of Eberron: Rising, could you check to see if it was omitted from the original list and then subsequently added with Tasha’s? DDB might have just missed it if that’s the case.

 The App hasn't been updated yet with the errata so I was able to read Eberron there. The Necklace of Adaptation was added in Tasha's. Some items may appear to have been removed from the Replicate Magic Item lists, but only because they are now covered under "any common magic item in the game", whereas Eberron's text specified common magic items from Xanathar's. The Arcane Propulsion Arm was given its own infusion separate from Replicate Magic Item.

Exactly one (1) item from Replicate Magic Item was removed completely, namely the Ventilating Lungs, which is probably the most Eberron-setting-specific item.

Necklace of Adaptation, once again not showing up as an option... Ventilating Lungs is available though. 

Did they never fix this or did this bug just return??? Because the Necklace of Adaptation no longer shows for me...

Quote from spamandtuna >>

Do you have 2014 Core Rules enabled for that character?

 
This bug has existed before any 2024 books were released. As far back as 2021.

The Tasha's book lists Necklace of Adaptation for the Replicate Magic Item infusion, it is a 10th level option.

Making a new character, homebrew disabled, all other options enabled - choose Artificer, level it to 10 and under infusions for the 10th level box choose replicate magic item: the necklace will not be anywhere in the list. 

This has been reported multiple times before.

https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/89944-artificer-infusions?comment=5

https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/133789-artificer-replicate-item-infusion-missing-item

https://www.dndbeyond.com/forums/d-d-beyond-general/bugs-support/200672-artificer-infusions