This is telling you that you have an auto generated package.use file in that directory. Look in /etc/portage for a possible updated config file or files .cfg0000_package.use (note you may have a lot depending on how long it has been giving you that error). From a root or super user command prompt run: cd /etc/portage/ and then ls -al as then are hidden files. (filenames starting with a . are hidden)
dispatch-conf is a tool designed to help update config files.
