Why must the bootloader be “unlocked”? Before flashing a custom ROM like /e/ you must unlock the bootloader otherwise the bootloader prohibits the custom ROM to boot. Most vendors only allow their own stock approved software to boot.
Warning: Unlocking the bootloader will erase all data on your device! Before proceeding, ensure the data you would like to retain is backed up to your PC and/or a online drive.
Enable OEM unlock in the Developer options under device Settings, if present.
Connect the device to your PC via USB.
On the computer, open a command prompt (on Windows) or terminal (on Linux or macOS) window, and type:
adb reboot bootloader
Once the device is in fastboot mode, verify your PC finds it by typing:
Now type the following command to unlock the bootloader:
find the right command here (select your device and then click install, and search for the unlock command).
Ensure that your bootloader is unlocked by running
fastboot oem device-info