Apparantly this is in the macOS 27 Beta 1 release notes but in the Software Update section: Devices configured with Reduced Security mode might fail to install macOS 27 beta 1. Devices in this mode might also be prevented from allowing kernel extensions or disabling System Integrity Protection. (178942816) Workaround: Restore the device's security mode to Full Security via Recovery OS and retry the update. See this page for details. Never made it down to Software Update, was thinking it might be in an "Installer" section of the release notes...

I performed the following steps to get around the error (in this order): Enabled Full Security in Recovery Mode Used the complete/downloaded macOS 27.0 Beta installer rather than Software Update Logged in as full administrator for the install, rather then just elevating from my normal standard user account for the install. Thanks to everyone who provided suggestions. Hopefully this thread is helpful for others encountering this issue.

Okay tried to post some comments but they failed. So trying here. MacBook Air M4 is running macOS 26.5.1 Rebooted several times before attempting install. Had already tried the full install before posting here. SIP is enabled My apologies on not having this info in the first post. The copy/paste of the error message removed some of the original text that included my trobuleshooting steps. I will try booting into recovery and setting the security level as suggested to see if that helps resolve the issue. Thanks for your interest on this post.