Java Question Simple Library Compiler can't find AndroidX packages

Starting step: Compiling Java code.
D:\Development\Projects\sly_calendar_library_wrapper\app\src\main\java\com\example\slycalendarlibrarywrapper\MainActivity.java:3: error: package androidx.appcompat.app does not exist
import androidx.appcompat.app.AppCompatActivity;
                             ^
1 error

javac 11.0.1

Error.

public class Activity extends AppCompatActivity implements SlyCalendarDialog.Callback {

    private BA ba;
    private String eventName;

    public void Initialize(final BA ba, final String eventName) {
        this.ba = ba;
        this.eventName = eventName;
    }

    public void showDialog() {
        new SlyCalendarDialog()
                .setSingle(false)
                .setCallback(Activity.this)
                .show(getFragmentManager(), "TAG_SLYCALENDAR");
    }

    [USER=69643]@override[/USER]
    public void onCancelled() {
        //TODO: Raise BA Event
    }

    [USER=69643]@override[/USER]
    public void onDataSelected(Calendar calendar, Calendar calendar1, int i, int i1) {
        //TODO: Raise BA Event
    }
}