I keep got this error message on iPhone8:
"The developer needs to upload a build that supports your device's architecture (armv6)."
And the same version is ok to install on other devices (iphone7,6,ipad...).
So far I had tried
update mac to 10.12.6 and reboot
keep clean whole Xcode project
update crashlytics/fabric version
try Xcode 8.3 and 9GM
And nothing work for me.
This is a known issue that the team is working to fix. I'll post an update to this answer once we have it working. It's top priority for us and we're looking to have a solution out ASAP.
UPDATE:
This is fixed now. Beta installs work on iPhone 8.