Open the "Avatars" tab and copy-paste the Avatar ID from Unity into the respective input (you can find the Avatar ID for example in thePipeline Manageron your avatar root or in the Content Manager section in the VRChat SDK window). Enter a name for the avatar and then clickAdd. ...
If Unity or the SDK don't seem to be working right, restart Unity and open your project again, or go to the VRChat SDK dropdown in the top menu bar and select Reload SDK. Make sure the title bar ends with "PC, Mac & Linux Standalone <DX11>". If it doesn't, type Ctrl+Shift+...