Hi there 

Thanks for reaching out in the community! You can create a relationship line in your Entity Relationship Diagram by clicking and dragging from any dot on an object. You can drag the line to another shape and create a relationship to a specific field or release the cursor over empty canvas to auto-prompt adding a new entity object. For more information on how to create ERDs in Lucidchart please review our Entity Relationship Diagrams article from the Help Center. 

Are you currently unable to create relationships between objects in your ERD? Could you please send a screenshot of the issue just to make sure we completely understand what you are experiencing? Also have you created your ERD manually or have you imported it into Lucidchart?

Thanks!