Unfortunately, in QGIS 3.2, the functionality to run SQL queries against GeoPakages has not been fully developed as of yet in the DBManager. To carry on exploring SQL queries, we are going to focus on SpatiaLite. To do this, we first need to connect to a SpatiaLite database, which can be found in your Chapter 3, Creating Spatial Databases, samples download.
In the Browser panel tab, right-click on SpatiaLite and choose New Connection. Navigate to open_data.sqlite from the Chapter 3, Creating Spatial Databases, samples.
Once connected, you can expand the open_data database and see the layers and tables stored within it.