A Service Method Invalid error will occur if a service contact selected for service does not have the required pieces of contact information needed to serve the contact.
If you have created the contact, click on the contact to expand the details and add the missing required information. Otherwise, if the contact is not yours/your firms, you will need to deselect the checkbox next to the contact in order to be able to proceed. You will need to serve the contact using a different method if this contact does need to receive service.