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If you get the prefab does not exist error: It is because the avatar is missing scripts.
Here is what worked for me.
1. Click on the console button that should be located next to your project tab. You will find a bunch of red errors.
2. Scroll to the top and find the first error that states that a script is missing
3. Find the armature piece that is giving off the warning (stated in the error) in your hierarchy, then click on it.
4. Look in the inspector (should be on the right side of your screen) and you’ll see that there is a script, but no actual script attached. Go ahead and just delete it
5. Repeat until you have removed all scripts included in the errors, it may take awhile, this particular avatar has a lot of missing scripts.
6. Upload and enjoy your avatar!

Savimrai,
dont import shaders just sdk, bones

fukkursitepls,
ummm ok i mean if you dont like them just leave :0 not that big of a deal i bet your an avatar creator who is butthurt over the fact your avis got stolen

fukkursitepls,
someones butthurt...

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you cant add the poi when you upload otherwise it wont work so don't put poi i it other then that its a great model it would be better to get it from here if anything is messed up https://roseflower.gumroad.com/l/LexiFreeModel

says prefab doesnt exist

PREFAB ID doesn't exist is what it comes up with

gabrieldlc900,
Delete the shaders then add them back in, worked for me anyway

it said file not found when i try to upload it