It’s complaining, becuase it’s expecting an empty database, not one allready full of stuff.
I’m assuming that you want to upgrade phbb? And you want to carry over all of your old posts into the newer version? In that case you’ll want to restore your old files so that your forum is in a working state (old version). Next go to the phpbb website, and download the upgrade version and follow upgrade instructions.
Alternativly, I suppose you could clear out the database entirley, get the new install of phpbb working, and then import your entires back through my database restore - but that won’t bring back the layout you were using - just entires and settings. The upgrade would really be better.