I have Photoshop 2017 downloaded and have an adobe photography subscription. I'm getting a message my serial number is invalid though? I'd rather not update my OS given I'm on an older Mac. Any suggestions?
To bring the answers together: one idea is that you have a Photography subscription, which could run Photoshop 2017 if you had it. But, Photoshop 2017 isn't available. So we think maybe you downloaded Photoshop Elements 2017 instead. No, you can't run that.