For me it was similar to this answer https://stackoverflow.com/a/57361245/2149955 but just a bit different so sharing a separate answer for when you haven't installed the tools through unity hub.
In Unity:
Edit->Preferences->External Tools
See error messages "You are missing the recommended <TOOL>. Install the recommended version using Unity Hub"
In Unity Hub:
Installs -> My Unity Version -> ... -> Add Modules, Twirl open Android Build Support -> Check Android SDK & NDK Tools (I also checked OpenJDK)
Press Next -> Accept Terms
Wait for install to finish
After that finishes, back in Unity:
If you look in external tools again, the error messages will be gone
Assets->Play Services Resolver->Android Resolver->Resolve