Having the (relatively) newly added "VRC Impostor Environment" component on your Avatar when trying to generate an Impostor will result in the Impostor Generation to fail.
Expected Behaviour:
The Impostor Generation should not fail when the VRC Impostor Environment" component is added to an Avatar. It instead should change the skybox colours that are being used while generating the impostor.
The first picture shows the component added to my Avatar and the second picture shows the Impostorization failed: