"SDK tools directory is missing", because when you install your computer can not connect to the google servers (domain name server address google resolve a problem), you can not download the SDK tools from google servers skipped automatically installed when you install SDK tools this step (flash off).
Solution: Manually add the host file to use the google server IP address, domain name address mapping. Referring specifically to:
https://www.cnblogs.com/chy18883701161/p/10863879.html how to modify the host file
https://www.cnblogs.com/chy18883701161/p/10863776.html how to add google server in the host file Address Mapping
Description: When complete, need to restart Android Studio, download the SDK tools in the SDK manager.