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If the field was just recently created in your Mailchimp account it could be that the MC4WP plugin is not yet “aware” of the fields existence.
In your WordPress admin to to MC4WP > Mailchimp, and click the Renew Mailchimp lists” button to pull in the latest data from Mailchimp. Otherwise it can take 24 hours before the new field is synced in via background sync.
Hope that helps. If you have any questions, please let me know!