Final answer:
You are unable to delete a custom field if it is referenced in a formula, workflow, or used in a report, as its deletion can negatively affect other parts of the system.
Step-by-step explanation:
The condition under which you are unable to delete a custom field in a platform such as a Customer Relationship Management (CRM) system is when the field is being referenced elsewhere within the system. Specifically, if the custom field is referenced in a formula, used as a criteria in a workflow rule or process, directly included in a report, or used in code such as an Apex class or trigger, then you will not be able to delete the field as it may cause other parts of the system to malfunction or produce inaccurate data. It's also worth noting that while a field being part of a page layout or being a required field does not inherently prevent its deletion, you might still be prompted to remove it from those layouts or adjust elsewhere to ensure the overall system's functionality is not compromised before you can proceed with deletion.