The Salesforce API returns error messages when it can’t deploy metadata. Learn more about the error, what the cause is and how you can fix the problem.
FIELD_CUSTOM_VALIDATION_EXCEPTION The field must be populated with a value
This issue occurs when the named field is blank but a field validation rule requires a value in that field. When you update a record the entire record is checked for custom and system validation rules, even if you are not editing these fields.
The same error also happens when you are using Apex validations with the .addError() method.
If you are sure that you do not have any custom validation enabled and do not use .addError() methods, the other possible scenario for error is that you have a managed package installed that is using these methods.