Some bootloaders are locked by the manufactures to prevent the phones from booting unofficial firmware images.

As the excerpt already indicates, a "bootloader lock" is set by the manufacturer to keep users from "tampering to much" with the device. So with a locked bootloader, you usually can not install a or even a custom recovery.

Implementations of bootloaders differ, so "hacks" to unlock them are often pretty device-specific (and sometimes even only work with a specific version). Some devices are even not un-lockable, on others this step is not revertible (as they e.g. use "fuses" you need to "burn", e.g. the ), and again others permit you to switch between "lock" and "unlock".

A common method working on most devices is using to run adb reboot oem-unlock. For security reasons, this will include a (e.g. to prevent hackers from getting access to your data).

More details can be obtained e.g. from the following questions and their answers:

You might also want to check other questions tagged , there might be one specific to your device.

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