Maybe refer to this one:
Another "shortcut" [sometimes may not be short]:
1) open ${your_project_dir}/local.properties
you should already see SDK directory config there; add your NDK's manually with your own local downloaded NDK directory:
ndk.dir=your/own/ndk/directory
2) then you could sync it again
(the other way is to configure your studio IDE to your SDK and NDK locations from "File --> Project Structure"
Question: if your project does not have native code, it should not ask for NDK location though. could you confirm?
thanks