Fastboot flashing is not allowed

[piero]

Hello @sienna, Welcome to this forum.

@piero

you may want or need to flash first an appropriate Stock Vendor Firmware to your device as a sane base :
Instructions and Downloads :
.
Factory Images for Nexus and Pixel Devices | Google Play services | Google for Developers

Install /e/OS on a Google Pixel 4a - “sunfish”

[HOWTO] an Unified Install Guide Project for /e/

posts are editable as a Wiki. Here is the summary : (1) install the appropriate Flashing Tools (2) allow “Non Vendor OS” to be installed on the device (3) Check the communication between the PC and your smartphone (4) Download a custom recovery manager (5) Download an /e/OS build (6) Unlock the Bootloader (7) Flash a custom recovery manager on your device (8) install /e/ from your computer, using the “adb sideload” method or (9) Install /e/ without computer using the “Device-Storage…