As ALL of the plugins in the Studio Magic Suite are from different 3rd party developers, they may require that you register with their original developers. This can be further complicated as different developers use different registration and copy protection methods.

Phat is product of UJAM NOT Presonus. Have you been to their website? Posted of their forum?
Or contacted their customer support?

https://www.ujam.com/phat/

Furthermore Studio One Artist doesn't load just any VST plugins, these are special editions made to work with your DAW. If yow wish to run any and all VST plugins you must by the VST addon for Artist or upgrade to Pro.

That being said we can help you better now and in the future with a bit more info......if you will post your system specs in your signature, (see my sig), we will be better able to troubleshoot your problems. Without knowing what type of pc you're running, what OS you're using, what DAW etc....we often can't recommend a "simple fix"

If you have not already done so please READ THE MANUAL!