This statement in that tutorial is not correct:
A build template must be imported into a Godot project prior to exporting an Android build.
That's only required if you're using a custom template, which may not be needed in your case.
You can export to Android using the default templates that are installed (outside the project) using Editor >> Manage Export Templates...
If you don't need a custom template, disable Use Gradle Build here:
Project >> Export... >> Android >> Options >> Gradle Build
If you do need a custom template, change Gradle Build >> Target SDK to a version of the Android SDK that's installed on your platform. In my case, these are the installed SDK's:
$ ls ~/Android/Sdk/build-tools/
27.0.3 28.0.3 29.0.2 29.0.3 30.0.2 30.0.3