sign up error

After sign up , when I click the activation link, got this error:

Already Installed

You appear to have already installed WordPress. To reinstall please clear your old database tables first.

but if I enter this link again it shows that it is activated, then ask me to wait the password to send to me, but no password is ever received, if I enable the password at sign up plugin by wpmu dev and enter a password, it will work no problem, but still have the same problem if I don’t enter pasword.