If the contact is missing info for the custom field, you'll see this marked in their contact status. You can hover over the "i" icon to see which custom field the contact is missing.
You can then click on the contact's profile and edit it 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 can clear the error for the contact again.
For clearing errors for multiple contacts in bulk, you can select the select all checkbox.
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.