I need to create user with username "user" but Windows 10 won't let me.

I delete user folder in users folder and open:
Settings->Accounts->Family & other users->Add someone else to this PC->(click on link)The person I want to add doesn't habe an email address->(click on link)->Add a user without a Microsoft account
And then I enter user name "user" and password and it saves it without problem. But I can't login like that user, I can't even see that user anywhere. Then I repeat all above procedure but then I got an error that I must type in other user name then "user".