When I click on the link to take me to the Adobe 'install flash player' page, I briefly see the install button before it disappears and is replaced by, "Note: Your antivirus software must allow you to install software."
As far as I'm aware I don't even have any anti virus software installed.
I'm on Mac OSX 10.11.4
Already tried the adobe trouble shooting options
Could you please install Stand alone Flash player from below link :Installation problems | Flash Player | Mac
so it will resolve the issue.
Debugging purpose ;
We will look into this issue right now, can we know in which browser you are facing this issue . so it will help us for further verification.