Q: why i cannot verified my disk

There are a number of different potential causes. Read Burner - device failed to calibrate the laser power level for this media for some context.

The optical drive uses laser diodes whose failure mode is characterized by gradual degradation to the point an optical disc can no longer be reliably burned. You might be able to get some more life out of it using the various methods in the above Discussion, but the right way to fix the problem is to replace the drive. They are not terribly difficult or expensive to replace.

Apple may or may not stock the required parts any more. You would have to ask: http://www.apple.com/retail/geniusbar/