Craft is unable to determine if an update is available at this time
You're getting that error message because there is an existing craft/config/license.key file in place for your local installation and there is no record of that install on the Craft/web service end.  Likely you've just copied an existing license.key file from another site into this new installation.

So the web service on the Craft end thinks that since it's a new Craft client installation it needs to send down a new license.key file for you to start using.  But the Craft client knows it should never overwrite an existing license.key file, so it reports that error.

Solution: remove the existing license.key file (since it probably belongs to another site anyway) and let the web service generate a new one for you.