Not being able to use the username and password generated by the WordPress system is the most common problem reported by users. “It won’t let me log-in”, they say.
90% of the time it is because the user is typing the details in wrong when they try to log-in.
My top tip is to copy and paste the password from the email you receive directly into the password filed on the log-in page. make sure you don’t get a trailing space by accident.
Once you are in, go and change your password immediately. Users -> Your Profile.