Cheers @amb!
As you’ll see from the docs @glv linked, Processing Python mode won’t work with the latest Processing 4 IDE, it is unmaintained, but it should still work with the Processing 3 IDE.
I’d like to stress it does not make use of the Python interpreter you installed, it will use its own special Jython 2.7 interpreter.
I’d also invite you to check out a new blend of Processing and Python, the py5 project that @glv metioned. I’ve been using it in classes along with Thonny IDE, here you’ll find some installation suggestions and a link to a portable version of Thonny with everything preinstalled:
How to install and start using py5 (with Thonny IDE)