Please let me know if you have any other Adobe application installed on your machine.
If yes, then please check the permissions for the folder under ~/Library/Application Support/Adobe
Right-click on the Adobe folder and choose "Get Info" from the menu, expand the Sharing and Permissions tab at the bottom of the window and check that your user account has Read & Write permissions.
Also, ensure that if there is a previous version of Reader installed on your machine, then uninstall it completely and then install Reader DC from here:
Try launching Flash Player as well and see if it works fine or not.
Let me know.
Apart from the Flash Player, I have Photoshop CS6 installed on my computer. The thing you said about Sharing and Permissions, I had already tried. I do not need Adobe Reader anymore but I still cannot update the Flash Player correctly through the website (it gives me the same problem as before).
Thanks for your help,
I already tried to do that and it did not work.
It seems to be the file & folder permission issue in Mac, please refer to this link for help Permissions & Disk Space | Apple Support Communities.
Once you have rights to install the software, please download Adobe Reader using this link Adobe Acrobat Reader DC Install for all versions.
Let me know if you experience any issue.
I am also moving this thread to the Flash forum for further help.