If a contact is missing info for a custom field within the sequence, you will see this marked as a Missing Info error in their status.
To see which custom field the contact is missing, hover over the gray "i" icon:
You can then click on the contact's profile and click Edit to add in the missing info:
Once you add the info for the contact, the error should be cleared.
If the error doesn't clear after saving the info for the custom field, then you will need to manually clear the error by clicking on their status again, then click Clear Error:
To clear errors for multiple contacts in bulk, click the Select All checkbox:
Please note that there is no option to edit missing info in bulk so you must open each contact separately. You can only manually clear the errors in bulk.
Note: If the custom field doesn't exist in the sequence, it can also trigger the missing info error so you can remove the field from the email step and clear the contact error.