I've found a very similar forum thread from last year that has the same DLL not found exception referenced in the user's error logs.
Try this first prior to reading the other solutions offered in the thread above:
Can you please verify that there are no residual versions of libovravatar.dll or OvrAvatarAssets.zip in your Unity project's Plugins directory?
Also, can you check to see if libovravatar.dll exists in your "Program Files\Oculus\Support\oculus-runtime" directory?
the libovravatar.dll is not even part of the project at this point. Why is it referenced or what is missing to get it in there?
DllNotFoundException: libovravatarOvrAvatarSDKManager.Initialize () (at Assets/Oculus/Avatar/Scripts/OvrAvatarSDKManager.cs:57)