You may not have done anything wrong at all! This infamous error message is a known phpBB problem which has proberly caused more heartaches than anything else, so first of all let us make it clear it's not caused by phpBB
There is no one simple answer, this error can be caused in a number of ways, so here is a few possible solutions, they have worked for others, maybe they will work for you too.
- Clear your browser cache and cookies, and fix cookies in your board, use Auto Cookies 1.0.6 to fix.
- Check all files have uploaded correctly, checking that your FTP client properly handled the .PHP & .TPL files as ASCII transfer (not BINARY).
- Delete and re-install the includes directory -- This error is connected to the includes/constants.php file in some way, or use this MOD to Fix message_die for multiple errors MOD 1.0.3
- ANALIZE and REPAIR data base of your board.
- Are you sure delete folders contrib/ and install/ ?