This must be an issue with any of your apps. I use this ROM on two devices (my wife's and mine), both running for at least two days on battery.
Hint: Don't use any battery saver app!
Points to check:
- latest ROM version
- latest Mi Unlock Tool
- Mi account details entered in phone
- some users had to wait up to ten days. I personally unlocked straight after receiving the message without any problems.
When did you receive the unlock permission? Some users had to wait up to ten days to be able to unlock.
Did you enter your Mi account detsils in Settings - Mi account? You'll have to use the same account as in the unlock tool.
Did you download TWRP image file and rename it to recovery.img?
I recommend 3.0.2 by cofface https://forum.xda-developers.com/xiaomi-redmi-3s/how-to/share-twrp-3-0-2-developed-cofface-t3471201
You can only unlock bootloader, if your device runs the latest official ROM. I think you'll have to make a deep flash cable to install an official ROM.