If your Asus Zenfone 6 is not being recognized by AndroidStudio, try the following steps:
1. Fetch the drivers from here (select ‘Android’ and then expand ‘USB’).
2. Extract the zip file.
3. Connect your phone to the PC.
4. Go to Device Manager. Find your device (should be under ‘Other Devices’ as ADB device).
5. Right click on the device and click install (or update) driver.
6. Browse to the location where you unzipped the downloaded drivers. Choose the appropriate folder and proceed with installation.
7. Once the installation is done, you will see your device as ‘ASUS Android Composite ADB Interface’ under ASUS Android Device.
8. You will see a confirmation popup on your phone. Accept it.
Now AndroidStudio should recognize your phone.