If you attempt to build for iOS or tvOS using the Default-Skybox material as a Skybox Material, the build will fail with the following error:

"BC6H compressed textures are not supported when publishing to iPhone"

Steps to Reproduce:
1. Create a new project
2. File > build settings
3. Switch platform to iOS
4. Build

Result:
iOS Platform: Error thrown in console "BC6H compressed textures are not supported when publishing to iPhone"
tvOS Platform: Error thrown in console "BC6H compressed textures are not supported when publishing to tvOS"

Expected Result:
The build completes successfully.

Environment:
Windows and Mac

Occurring with:
2018.3.0a5

Working with:
2018.3.0a4