I'm trying to set up my repository to use an older version of Python for compatibility.
I was able to select my system Python interpreter from this menu:
How do I tell if I'm using a virtual environment Python interpreter or if I'm using my system interpreter?
Because I need to install some packages from the requirements.txt and don't want to mess up my system Python.
In your first image it shows in the left menu that you are using an existing virtual environment based on your system's Python installation (which is typically the default in PyCharm). You should change this to "New" if you want a project-specific environment, and then it will give you the option of selecting the associated path (by default it is in the project folder). You would select "System Interpreter" on the left menu if you did not want to use a virtual environment. You can retroactively change the project settings if needed.