Can't install program

I'm trying to install Cisco AnyConnect Secure Mobility Client. I need it to access webpages through my uni's vpn. Every time I download it and try to install it through the client installer it asks me for the install destination but I can't choose Macintosh HD because it says it is already installed. I have definitely never installed it on this device before because I only just bought it 2 months ago but as far as I can remember I've never installed it on any other device either. Quick responses would be really helpful, I can't do school work without the vpn. Also I am updated to the latest software and I have more than enough storage space left. Thanks in advance :)