The following are possible scenarios in which you may encounter errors with custom field mapping changes in LinkedIn Sales Navigator: 

1. The admin has lost read access to one or more of the custom fields that has been used for mapping
2. A custom field that was used in the mapping has been deleted in the CRM 
3. The CRM is disconnected from Sales Navigator  
4. A user doesn’t have write permission to the field

If either of the first two scenarios occur, when you navigate to the Custom Field Mapping page under Admin settings, you will notice Not Available displayed in the CRM field column. Also, you will notice the following error message displayed under Admin Settings: 

Custom field(s) used for CRM Sync are missing.

This message includes a Review option that you can use to determine which of the custom fields are missing or which fields the admin or user has lost permissions for.

To resolve these errors, you can either use the Custom Field Mapping tool to map entities to existing entities in the CRM, or you can login to the CRM and rectify admin and user permissions. 

Learn more about custom field mapping:

• Use Custom Field Mapping Tool for CRM Entities