I had the same problem with MI Redmi 4a, I solved it →
first become a developer by clicking
Settings-> About Phone-> MIUI Version (7-8 times)
then setup-> advanced setup-> developer options-> USB debugging
Enable usb debugging, then go to the developer’s settings for the last item name: Enable MIUI optimization ", disable this option, it will ask you to reboot, then reboot the device by clicking the Disable and reboot button
after rebooting go to Settings-> Permissions click the button in the upper right corner. Enable install via USB option
here it is!
dharmx Sep 28 '17 at 12:12 2017-09-28 12:12
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