Adding a follow up campaign from someone who has been previously removed is not allowed, primarly because you could be adding back someone that has been asked to stop receiving emails which is against spam laws. Secondly, if that person were to have received the first 4 emails in a 10 step campaign we’d restart them at the very beginning again.
If you are testing, then I suggest creating another contact record and use that as your new test contact record. Knowing this rule will help you as you run your tests. Keep in mind that you can test run any email directly by going to a contact record, clicking on the email account you’d like to send the email to, then from the email compose window click the email templates icon and manually select the email template (that will also be in the follow up campaign), then click send email. This will be the exact same email that will be sent from the FUP campaign with the exception of the unsubscribe footer.