Receiving Error When Creating Lookup Field on an Object(s)

The error occurs because the 'Add Related List' option triggers an internal conflict on certain objects. To successfully create the Lookup field, disable this option during field creation and add the related list manually afterward.

1) Navigate to the object where you want to create the Lookup field (for example, Work Order) in Setup > Object Manager.

2) Click Fields & Relationships, then click New.

3) Select Lookup Relationship as the Data Type and click Next.

 
4) Choose the related object for the lookup (for example, Visit) and click Next.
 
5) Fill in all required fields and click Next.

 
6) Adjust Field-Level Security as needed and click Next.

 
7) On the Page Layouts step, assign layouts as needed and click Next.

 8) Important: Before saving, uncheck the Add Related List checkbox, then click Save. This prevents the internal server error from occurring.

 
9) After the Lookup field is created successfully, manually add the related list back to the object's Page Layout.
 
After the lookup field is created successfully, manually add the related list to the object's Page Layout.