Hi @AME welcome to the /e/ Forum.
One possible reason could be if the phone was already running Android 15 which has been available since about October 2024 on shiba. The manual install page Install /e/OS on a Google Pixel 8 - “shiba” tells us
Before following these instructions please ensure that the device is on the latest Android 14 firmware.
Did the /e/OS Installer give any such warning?
If this were the case one would have to downgrade Android version … this page Factory Images for Nexus and Pixel Devices | Google Play services | Google for Developers will help with ROMs and method. You may be able to use Android Flash Tool.