Official LineageOS install instructions for the Nextbit Robin do not reflect reality and are non functional. Here is the proper way to get it done:

Unlock the Bootloader:

Install USB driver for Nextbit Robin: link
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
You can also boot into fastboot mode by powering off the device and holding Volume Up + Power
With the device powered down, hold the Volume Up and Power buttons.
Now type the following command to unlock the bootloader:
fastboot -i 0x2c3f oem unlock-go
If the device doesn’t automatically reboot, reboot it. It should now be unlocked.
Since the device resets completely, you will need to re-enable USB debugging to continue.

Installing a custom recovery using fastboot

Download a custom recovery – you can download TWRP. Simply download the latest recovery file, named something like twrp-x.x.x-x-ether.img
Connect your 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
You can also boot into fastboot mode by powering off the device and holding Volume Up + Power
Flash the recovery:
fastboot -i 0x2c3f flash recovery twrp-3.1.1-0-ether.img
Unplug the device and turn it off by holding the power button.
Boot the device up using Volume Up + Power button and select recovery.
Format the device and wipe partitions
Copy over LineageOS, opengapps, and ARM64 Superuser using usb mount or adb push filename.zip /sdcard/
Flash the .zips (LineageOS first)

If you encrypt your device you will not be able to open it with twrp to flash new builds. You can use Flashfire to flash updates without a recovery.