Please be sure that all first and last names are both entered in the correct spaces. Leaving a blank space for either the first or last name will cause errors when uploading your guest list.
It's possible that you are running into this issue is because you have included addresses, but left out phone numbers and email addresses.
When you include the address, it activates the other fields, which are also required. You will need to include your guest phone numbers and email addresses in order for this import to go in successfully. Otherwise, you can just import your guest names and leave out the address, as guests only need to search for their names in order to RSVP.
For large guest lists, it can help to import groups of less than 50 at a time.
If you continue to experience this issue, please let me know!