Hello @cjcardoni ,
Are there any accents in the name - á, à, ä, é, è, ê, ë, í, î, ï, ó, ô, ö, ú, û, ü or ç ? When these are part of an email address it will result in the address not being accepted, even when the actual first name and last name fields are due to how coding for email addresses works with several functions of accounts. If the address has an accent, then you'll need to get a different email address from the contact to utilize.
If the address doesn't have accents, check your unsubscribed contacts to see if the address is in there. If you're trying to add a single contact, and they're already unsubscribed from you, it can result in the single-add method erroring out.
The other troubleshooting method would be to add the contact via the type/paste multiple or file import methods.