When creating a preview test in Email Insights and choosing to run a spam test with it, you will need to select a database to use for the test.
You might only have one database to choose from in the drop-down menu. This is because that is the database (or parent database of a query) currently selected as the contact source for your template.
To select a different database, you will need to make sure no contact source is selected in the template. Having the template configured like this will then display all of the databases that can be used for this test. If you already ran a preview test with a spam test and want to retest with a different database, you can. You can either select a new database (if no database is applied to the template) or you will need to change the database applied to the email.