Which Android version runs on your phone?
My Android version is 2.3.6. I bought my Samsung very recently.
it's effectively a very outdated phone.
while the device itself hasn't been on the market for a very long time, it's specifications are similar to phones from over 3 years ago.
if you expect to run heavy applications like photoshop you really need a phone with android version 4 or newer, and a 1ghz dual core cpu or better. (not based on actual minimum requirements for photoshop specifically, just a recommendation in general)
Photoshop Touch for phone needs Android 4.0 and newer. That it's why it's shown as non compatible in the play store. Also, as already has been mentioned, the phone is probably lacking in hardware capabilities.