You can produce a "D's and F's Report" based on the Final Grade column in the Gradebook. Just be sure to select (1) the Students module, (2) the Subjects module, as well as (3) the Gradebook module:
From there, be sure to select at least the following fields:
- Something from the Students module, like Student Name and Student Number
- Something from the Subjects module, like Subject Name or Section #
- "Final Grade (letter grade)" from the Gradebook module
Finally, when running the report, you'll want to create a filter on the "Final Grade (letter grade)" field to only show D's and F's. Your final report with filters should look something like the following: