Sounds like it's given you a pointer as to the area of the import that is invalid.
I don't know how many users you are trying to import but perhaps to trouble shoot this issue you split the users up if it's a mixture of domain and none domain users.
Are you trying to load users from a domain nprinting doesn't have access to?
Are you using the wrong syntax to load the domain user
Thanks for your reply, Andy!
The Domain Account syntax was correct. I was importing around 500 users.
I removed the users that had special any characters(., comma, !, _,- ,:) in their Name and also completely removed the entries for column 'Nickname'. Then I was successfully able to import the users.