The WHERE clause from the main SELECT statement cannot use "=" operator for a multi-query. Because the sub SELECT statement returns multiple rows, you must use either ANY, ALL, IN, or NOT IN to specify the values to compare in the main WHERE clause.
For example, the example query must be
SELECT d.location_ID
FROM departments d
WHERE d.department_ID IN (SELECT e.department_id
FROM employees e
WHERE e.manager_ID = 100)
In the Query Builder adjust the WHERE clause with the appropiate operator.
Note: Query Builder will not check for syntax convention please make sure you consult a PL/SQL text for the appropiate syntax to multi-queries.