Microsoft recently removed their Windows 10 19H1 ISO, meaning Insiders had no way to clean install the OS for testing purposes.
The good news is that the downloadable version of the OS is now back.
We have released new ISOs (with an updated expiry date!) for download @ https://t.co/l8663ySB0P.
NOTE: this was not a Slow ring release, it only went to Fast. We wanted to provide new ISOs to aid users are having trouble updating and may hit the build expiry scenario.
— Windows Insider (@windowsinsider) December 11, 2018
Insiders who wish to install from a thumb drive can now download Windows 10 build 18290, which was released to the Fast Ring a few weeks ago. Read more about what that brought here.
In the meantime you can download the official ISOs here.