How to reproduce:

1. Download 2019.3.0a4 trough Unity Hub with Android + NDK/SDK tools, notice that r16b is downloaded

2. Try to do Android IL2CPP build and notice that Editor complains about NDK tools not found

3. Download latest NDK r19 version from Android developer page - https://developer.android.com/ndk/downloads

4. When setting to the newest NDK, you will get the error message "NDK 19.2.5345600 detected. Unity requires NDK r19 (64-bit) (19.0.5232133)."

Reproducible with - 2019.3.0a4

Not reproducible with - 2019.3.0a3

Workaround until the correct version will be downloaded via Hub is to download r19 version from the developer.android directly. Just follow the steps to download the latest ndk r19 version (currently it's r19c) and once you'll have the download link, just delete the letter after the version number, so the link would have only ndk-r19.

Fixed in Unity Hub 2.0.4 version