This happens because the android-28 SDK is missing; just install it.
The commonly suggested approach is to install Android Studio and then use the SDK Manager to install the missing SDK version. But that is too much trouble, so here is a simple method.
As it happens, when you install Unity and tick the Android support option, an Android SDK Manager is downloaded by default. We can simply use this built-in SDK Manager to install it.
What is more, this tool is a command-line tool, so we just type a command.
First, open a CMD window with administrator privileges;
Then, enter the following command:
"C:\Program Files\Unity\Hub\Editor\2019.4.30f1\Editor\Data\PlaybackEngines\AndroidPlayer\SDK\tools\bin\sdkmanager.bat" "platforms;android-28"
Remember to change the Unity path to the actual path on your own machine.
If the command fails, check whether the CMD window was opened with administrator privileges.