Did you installed the attendance module before? Stable versions of the module don't allow more than one instance on a particular course. If that's not the case, you can purge the database from any attendance related data and start the installation from scratch:
- Do a database backup so we can undo the changes if we screw anything.
- In 'mdl_modules' look for 'attforblock' in the 'name' field and delete the record.
- Delete all the tables whose name starts with 'mdl_att'.
- Log in as an admin and select the first option in the admin menu ("Notifications"?). That should reinstall the module from scratch.
I hope it works for you...