Sorry I may have convoluted two issues. One is trying to build ROOT for a system where it is entirely disconnected, which I believe you have provided enough information to track down.
Another issue is when a single package is unavailable from the internet. This has occurred int he past for me with TBB:
My question was if it is possible to have CMake not stop on these sort of failed downloads and instead just provide a warning and a flag that an optional package wasn’t available and then continue with the build.