If a user cannot login, either their email and password is incorrect. If they attempted 5 times in a row unsuccessfully, then their account will be locked out from any further login attempts. Another reason is the user may be archived. Both of these scenarios will be detailed here.
If a user has attempted 5 unsuccessful login attempts the user cannot login. Uncheck the box to allow the user to try again. As the user may not know the password, you may want to change the password and let this user know what the correct password is.
If a user is archived they cannot login. This is used when a user is not part of your company anymore and they do not need to use the system, but you want to keep the data for historical purposes. Once you archive a user it will free up slots for additional users.