Unlock the bootloader of Google Nexus 5X
Step 1: Download and install Google ADB Driver on your Windows Computer.
Step 2: Now, you have to Enable USB Debugging on Your Nexus 5X. To enable Developers Option: Open Settings > About Phone > Tap on the Built Number for 6-8 times until developers option is enabled.
Step 3: Now, Again Go Back to the Settings > Developers Option > Enable OEM unlocking and USB Debugging.
Step 4: Now, Power Off your Nexus 5X smartphone.
Step 5: Now, you need to boot into the Fastboot Mode. To Boot into fastboot mode: Press and Hold Volume Down + Power Key until fastboot mode is enabled.
Step 6: Now, Connect your smartphone to the Computer.
Step 7: Download and install Minimal ADB and Fastboot Tools on your Computer.
Step 8: Once, Minimal ADB and Fastboot Tools is installed on your computer you need to launch it.
Step 9: Now, in the Minimal ADB and Fastboot Tools CMD Window, Give the following Command: fastboot devices
Step 10: Now, Give another Command: fastboot oem unlock
Step 11: Now, you will get a confirmation screen on your smartphone screen, asking you to confirm this action. You can confirm using the Volume Keys to Scroll and Power Key to Confirm.
Step 12: After Confirmation, again give another command in the Minimal ADB and Fastboot CMD Window: fastboot reboot
Thats All. Your device bootloader is successfully unlocked. Now, you can flash any custom recovery on your device without having any issue.