In addition to the normal Activity Settings, you have an additional options for Pathways with Diagnostic Assessments.
When adding individual Activities to a Pathway which contains a Diagnostic Assessment, if one of the Activities does not link to questions in the diagnostic Question Bank, it will not be included within the Diagnostic Assessment and will be marked as an exempt Activity (unless the user chooses to skip the Diagnostic Assessment and complete all eLearning). Therefore, you have the option to mark the Activity as Must Complete, making it mandatory for the User to complete it.
Navigate to the Activity Settings area (see here if you're unsure how to do this).
Select Must Complete to ensure the User completes the Activity as part of the Pathway: