I'm sorry to make this an issue but I've been banging my head against a wall for nearly a month trying to figure this out now. When using a datapack to add the elytra flight power to an origin, you are supposed to be able to tell it what texture to render with.

"type": "origins:elytra_flight",
"render_elytra": true,
"texture_location": "minecraft:textures/entity/elytra.png"

I cannot for the life of my get it to direct to any other texture without it telling me the specified file doesn't exist with a "failed to load texture, file not found" error. I've tried loading it from within the datapack in almost every folder structure I can think of, tried loading it from a texture pack in several different folder structures, no luck.

I made a pastebin with the most recent iteration of the error, in case it's actually an issue and not me messing up the file syntax. I know this might not count as a proper issue but I am getting desperate. Pastebin here of the Failed to Load Texture error. This one was trying to put it in a texture pack and load from the specified location the same way you would a default texture, but its been the same error every time as far as I can tell, no matter where I put it/what I try.

The texture needs to be in a resource pack, using assets/<namespace>/<path>
In the case of your log, that would be assets/minecraft/textures/entity/phoenixelytra.png

So, I was pretty sure I'd tried it as that before, but I tried it again just to be sure, literally copying the file path you gave me there, and it is giving the same error.

[15:39:50] [Render thread/WARN]: Failed to load texture: minecraft:assets/minecraft/textures/entity/phoenix_elytra.png java.io.FileNotFoundException: minecraft:assets/minecraft/textures/entity/phoenix_elytra.png

The texture is called phoenix_elytra.png and is located in a resource pack following exactly that file path. I also tried it on a private server a friend lent me with a required server texture pack for good measure and got the same error. I have now tried disabling any mods I had that I thought might have been messing with the rendering of the player (3dSkinLayers, Ears, and Optifine) and no luck. I plan to continue disabling mods to see if any of them are causing this.

Sorry, the associated path would've been assets/minecraft/textures/entity/phoenix_elytra.png for the location minecraft:textures/entity/phoenix_elytra.png

In the case of raw textures the asset path is assets/<namespace>/<path> for the location <namespace>:<path>

I was so sure I had tried that syntax too but apparently not. Seriously, THANK YOU, I've been tearing my hair out over this for ages