Xiaomi phones come with a locked bootloader.
Device with locked bootloader means you will be not able to flash Fastboot ROM as well as third party custom ROM.
Here is the step by step guide to unlock the bootloader of any Xiaomi phone:
Step 1: Open browser and visit official website.
Step 2: Click unlock now button, you will get directed to login page. Put your MI login credential here.
Step 3: Fill in all details mentioned as given in below image. Don’t forget to give a proper reason for unlocking (Don’t copy & paste available reasons on the internet as it is). Also, submit correct phone number.
Step 4: Once you submit a request you will get a message something like this. After this step just relaxes and wait for a confirmation message from Xiaomi for approval of the request.
Step 5: Click here to download Mi Unlock Tool. After downloading install it on your PC.
Step 6: Ensure you have logged into your Device with the same account for which you have granted unlocking permission.
Step 7: Power off your device, then press + hold power key & volume down key for few second and then release them, it will help you to enter into “Fastboot Mode” on the device.
Step 8: Now run MI Unlock tool with administrative privileges on your windows PC, sign into unlocking tool with the same credential for which you got unlock permission and with which you have signed into your device.
Step 9: Be sure you are connected to the internet as unlock tool will check your account permission with servers and then go to next window as shown below.
Step 10: Now connect your USB cable one end to your PC USB port and other port to your device which is in Fastboot mode.
Step 11: Once your phone get connected to unlock tool, hit unlock button at the bottom of the tool, it will verify few details while progressing and eventually your device will be unlocked. This process will take 10-15 seconds after which you will be having your device unlocked. Then hit reboot phone button, your phone will be rebooted with bootloader unlocked.