Rather than start a new thread on this, I thought I'd add my experience here although it seems the OP problem is resolved. I found this thread doing my own troubleshooting so maybe someone will similarly wander upon it.
I downloaded the Public Beta and on launch got the message:
"This copy of the Install macOS Sierra Public Beta application is damaged, and can't be used to install macOS."
I downloaded it twice more to the same irritation. Grrr
In the end I used Disk Maker X to create a bootable install drive (it hasn't been updated for Sierra support and it did throw up some error messages while running, but it did still succeed in creating a bootable drive). I installed the OS successfully from there.
Still no clearer why the download said it was screwed. Only thing I can think of is that my internal drive is fried so perhaps the installer failed when encountering something it wasnt expecting? (speculation)