Issue is due to the fact that lineage 14.1 bullhead nightlies are based on the 7.1.2 N2G47F (Apr 2017) monthly update from google. More explations in xda-developers.

This assumes:

• you already know a bit about flashing you android device and you already installed TWRP.
• you already have adb / fastboot installed and you know a bit about it
• you are on macOS (I give some terminal command from macOS)

Download from official Nexus 5x N2G47F image

In case link is broken find it yourself:

• https://developers.google.com/android/images
• search N2G47F (CTRL+F - CTRL+V)

In terminal, check downloaded zip checksum:

shasum -a 256 bullhead-n2g47f-factory-f5d53a67.zip

it should be: f5d53a67662d1b4c2b2325813d823fc49ad9fb4a5b5361e91c3aff00157c1ddf bullhead-n2g47f-factory-f5d53a67.zip.

Then enter extracted directory:

cd bullhead-n2g47f

Then enter extracted directory:

cd image-bullhead-n2g47f

You can manully copy it to sdcard or by adb:

adb push vendor.img /sdcard

Manually by holding buttons: power + volume up + volume down, or by adb:

adb reboot bootloader

In TWRP:

• install vendor.img as vendor (check it when asked)

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