Cannot be installed on 2021.2.15f1 (USDU-246)

[MikeKotys]

The package cannot be installed on 2021.2.15f1. Cannot be found in Package Manager (with pre-released packages enabled) and cannot be installed using the git clone url.

[nicolaspopravka]

True. But if you can build the USD Unity SDK from source, then you can add the package from disk in the Package Manager. The 3.0.0-exp.2 USD package can then be added in a 2021.2.15f1 Unity project.

[cannon]

Try installing this: https://github.com/Unity-Technologies/usd-unity-sdk.git?path=/package/com.unity.formats.usd

This really needs clearer documentation...

[vickycl]

The package is experimental, rather than pre-release (see distinction here: https://docs.unity3d.com/Manual/upm-lifecycle.html)
As of 2021.1, Unity no longer surfaces Experimental packages in the Package Manager (see here: https://docs.unity3d.com/Manual/pack-exp.html)

Hopefully the workaround above worked for you, you should also still be able to install the package by name in PackMan (Add Package by name... > "com.unity.formats.usd")