We have heard from a few users that they're not able to find PHAT in Studio One Artist on Windows.
If this happens to you, here's a little trick provided from the PreSonus team:
You can trash your plugin settings folder and try again.
To do this:
Open the Studio One Settings folder via the Help Menu in Studio One.
Then close Studio One.
Select the x64 folder and delete it.
Now start Studio One again to have the plugins rescanned.
For other questions around Studio One please also check this article: How to Set up UJAM Instruments Plug-ins in Studio One