I've created a datapack with the purpose of giving the player a different item based on the origin he chose, via Advancements. My modpack is in 1.16.5 forge version 36.2.34, and uses this version:
Origins-1.16.5-0.7.3.9-forge.jar

After the player chose his origin, the datapack doesnt give him any item, even tho the file format is correct. I tried running in with fabric and Origins-1.19.2-1.7.1.jar, and there it works.
Datapacks are supposed to work on both versions, so what's the problem? Am I missing something?

Here is the datapack (not complete, but enough to test its purpose).
Drafts.zip

Note: the item the datapacks gives you comes from this mod (EverPotion), so if the mod is not installed it's normal that you don't get any item after successfully choosing your origin.

If you were using 1.19.2 powers on 1.16.5, where they don't exist yet, that's probably your issue here.

I misread your issue.

I think that the origins:chose_origin advancement trigger does not exist on 1.16.5. You may be better off using an Action on Callback power, to achieve the same effect.

https://origins.readthedocs.io/en/0.7.3/

Here's the docs for 1.16.5 specifically.