Flash boot.img pixel 3a

 $ adb devices
List of devices attached
* daemon not running; starting now at tcp:5037
* daemon started successfully
9CPAY1Q9QH    device

dhwan@dhwan-Aspire-A514:~$ adb devices
List of devices attached
9CPAY1Q9QH    device

dhwan@dhwan-Aspire-A514:~$ adb reboot bootloader
dhwan@dhwan-Aspire-A514:~$ fasboot devices

Command 'fasboot' not found, did you mean:

  command 'fastboot' from deb fastboot (1:8.1.0+r23-5ubuntu2)

Try: sudo apt install <deb name>

dhwan@dhwan-Aspire-A514:~$ fastboot devices
9CPAY1Q9QH    fastboot
dhwan@dhwan-Aspire-A514:~$ fastboot flash boot magisk_patched.img
target reported max download size of 268435456 bytes
sending 'boot_b' (65536 KB)...
OKAY [  2.210s]
writing 'boot_b'...
FAILED (remote: Fastboot command (flash:) is not allowed when locked)
finished. total time: 2.343s
dhwan@dhwan-Aspire-A514:~$ fastboot flashing unlock
...
OKAY [  0.136s]
finished. total time: 0.136s
dhwan@dhwan-Aspire-A514:~$ fastboot flash boot magisk_patched.img
target reported max download size of 268435456 bytes
sending 'boot_b' (65536 KB)...
OKAY [  2.148s]
writing 'boot_b'...
OKAY [  0.433s]
finished. total time: 2.580s
dhwan@dhwan-Aspire-A514:~$ fastboot reboot
rebooting...

finished. total time: 0.110s