Run the query in the right panel.
To quickly fix the problem with the sub-select statement, change it to return only a single row using the Fetch First syntax. If you look at the SQL in the editor, you'll see that 'fetch first row only' has been added to the query. This tells DB2 to only return one row, which is exactly what the query needs the sub-select to return.
Running this new query results in the report working as expected.