Challenge is not complete yet : Couldn’t find the following users in the org as partners with the Partner Community license and Partner Community User profile: Sean Forbes, Rose Gonzalez, Babara Levy, Josh Davis.

I just had the exact same problem and it was linked to the fact that 2 of those users had a non active user because I wrongly created them in the first place. I had to disable them and re-enable them and when you do this the system disable a user and creates a new one. probably same reason as why it would work if you switch all the first and and last names. (I did what Tomohiro suggested but only on one to see if it would go away from the error)

Check your user list and if you find the same user but deactivated then go in those non active one and change the name, alias, and everything you can think of. Once done, it will work :)