Yosemite is no longer available for download, unless you had previously downloaded it, it should show up in your Purchased section of the App Store. As your MacPro 5.1 can upgrade higher than El Capitan you will be refused the download of it.
Your MacPro 5.1 can upgrade to Sierra which is still available for download and should run Adobe CS6 apps.
How to upgrade to macOS Sierra - Apple Support
See screenshot at the bottom of the reply..
If you are worried by app compatibility you can have different OS's running on different disks or partitions, your Mac has room for 4 internal drives. As it is a MacPro 5.1 it can upgrade to Mojave if you have a 'Metal' capable graphics card installed. So you can run your CS6 apps on one disk or partition on either Sierra or keep Mavericks, and have newer OS on another disk or partition. If you don't have a 'Metal' capable graphics card then High Sierra is available too, but may have problems with CS6 apps.
How to upgrade to macOS High Sierra - Apple Support
How to upgrade to macOS Mojave - Apple Support
So you can elect to keep your Mavericks install and run your CS6 apps on that, or upgrade to Sierra and run CS6 there.
On another disk or partition you can install any of the newer OS's, (Sierra, High Sierra and Mojave all receive security upgrades
anything older than Sierra no longer does) so you can run any web based apps on that disk or partition. When you download a new OS, before you go ahead with the install look in the Applications folder for an item called Install macOS xxxxxx.app and make copy of it and keep it safe on another disk or USB stick, the one in the Applications folder self deletes on completion of the install.
Drag the screenshot below onto your desktop and open in Preview for a larger clearer view.
