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First to the basics: make sure your Xiaomi account is well linked to the mobile and you have active USB debugging from developer settings.
See that you meet the minimum requirements to unlock the bootloader.
It is also recommended that you raise your account level.
If you have tried everything we have said and still you still get the same message you just have to try one more thing. You will have to get a new Xiaomi account and start the entire unlock process again.