I enrolled into WIP to get the latest build. I'm currently on 1809.
The error message says:
This PC can't be upgraded to Windows 10. Although you are enrolled in the Windows Insider Program your device is not yet eligible for this build.
Can't find anything about what might make my device ineligible. (I'm on a Lenovo IdeaPad Y570, which I believe is quite powerful enough)
The only button is a Refresh button that doesn't do anything. And the "Why am I seeing this" link is also useless.
I tried freeing up some additional space on my C: drive and now have 23GB free, but after a reboot I saw this and ended up with the exact same error as before.
EDIT: I found in C:\$WINDOWS.~BT\Sources\Panther\setuperr.log this line that might be the offending issue:
2019-02-11 18:37:17, Error CONX 0xd0000034 Failed to add user mode driver [%SystemRoot%\system32\DRIVERS\UMDF\uicciso.dll]