Install MIUI PlayStore using Google Installer, got error: can't downgrade apps for google play service
After struggling for a while, I finally successfully installed Google play store. The solution is very simple. We don't need Google installer anymore as Google play service is already installed in the later MIUI version (In my case is MIUI 11), we can directly install Google play store from the APK downloaded from APK websites such as apkmirror.com. After Google play store is installed.