Moving this discussion to the Using Flash Player forum.
I don't have enough information to give you a definitive answer.
My guess is that you're either subject to some administrative controls that prevent you from installing, or you have some security software or malware on your system that's preventing you from upgrading.
As a first step, check out this installation troubleshooting guide:
If the problem persists, please let us know what OS and browser you're using. A screenshot of the error might be helpful as well.