Scenario:
User is performing CSV Import on a Record with mandatory custom fields and is getting an error.
Solution:
Navigate under Setup > Import/Export > Import CSV Records
Click Select button for selecting CSV file
Click Next
Import options (2nd step):
Click Advanced Options
Validate Mandatory Custom Fields: Remove Checkmark
Click Next
Set the Field Mapping
Click Next
Click Save and Run
Note: The system will then ignore mandatory custom fields once the Validate Mandatory Custom Fields is unchecked