CS6 its likely to contain some 32bit code. Which may make CS6 not compatible with Apples latest Operating System versions. As far as I know Apple no longer support any bit code. You may need to run an old version of Mac OSX.
Adobe Photoshop went Subscription Only after CS6. There is no upgrade path or permanent licenses anymore. You can get it as part of the Photography Plan, starting at $10 a month for a 1 year commitment.