According to Vocaloid Wiki, Unity with VOCALOID only works with 5.0, so maybe it becoming incompatible with newer Unity is an issue. You can see this page for more info:How do you get Vocaloid with Unity to work? I have tried following the manual, but still failed

Unity with VOCALOID
Unity with VOCALOID is a version of the VOCALOID engine built for the Unity Engine, similar to how the software was adapted previously to create VOCALOID Editor for Cubase for the Cubase software. It was released sometime in December 2015. The software acts as a plugin for Vocaloid, allowing it...
Honestly, I don't think trying to get it to work is worth it. Unity-chan's voice is significantly worse-sounding in the SDK version (it's her VOCALOID 2 version). Also, after all these years, only like 2 people have uploaded their progress with it (on nico nico). I have scoured the internet in both English and Japanese and could not find any user-made guides how to use it. I think too much actual developing experience with Unity is required for it to be useful for people who can't already understand developing with Unity (since it has no interface/piano roll, you'd have to program your own).
If you haven't seen it already, addictedtoheroines has a video with a portion dedicated to Unity with VOCALOID, here is the timestamp for when the segment begins:
At 23:15, he gives warnings about how it works.
Clips from nico nico to demonstrate how she sounds is at 24:35-27:13.