Once the target phone is in Fastboot mode (usually indicated by a "Fastboot" logo or text), check the connection again: fastboot devices Use code with caution.
While Termux is a powerful terminal emulator that provides a Linux environment on Android, it operates within the user space of the operating system. It does not have the deep system-level permissions required to write to the bootloader partition or communicate with the device's bootloader mode (Fastboot). unlock bootloader using termux link
This allows you to connect two phones together. Once the target phone is in Fastboot mode
Generic fastboot unlock flow (will wipe data and may display irreversible warnings): unlock bootloader using termux link
adb --version fastboot --version