The Android 8.1 beta update started rolling out late last week. Shortly after the roll out began, user reports started pouring in that the update is failing to install. Well, the good news is that the problem is now fixed. The confirmation came in the form of a forum post by a Google employee Orrin. To give you a quick recap of the issue, users said as they attempted to install the OTA update, the phone rebooted and got to the Google logo (this happened multiple times), following which it rebooted back to 8.0. The system then displayed a notification saying "Couldn't install system...
Source: https://www.gsmarena.com/google_confirms_android_81_beta_installation_issue_has_been_fixed-news-28010.php