My Mac Pro 5,1 mid 2010 with 2012 processors has the prerequisite for beta 2 - Metal capable Radeon 7970 and no FileVault. It rejected me on the installer after downloading it on another system. With this issue you also cannot download. I suspect the card is not seen as valid. I used the terminal prompt "sudo /Applications/Install\ macOS\ Mojave\ Beta.app/Contents/Resources/startosinstall" and it worked. Some issues with iMessage not letting me log in. I will try the old PRAM reset for that.