You put that in the wrong place. Sort of.
Well, you tried. To be fair, knowing where exactly to insert changes DOES require you to know how to read the code and be able to work out how the script itself is doing things. It's arguably half or more of learning how to script because even if you knew Ruby and all the syntax you would still need to read through the default codebase in the engine to know where to insert any additional features you wanted to code.
And when you are trying to modify someone else's script, you then need to read through their stuff and work out how it interacts with the default codebase. Depending on how big and messily it was coded, it CAN be that hard.
But anyway.
That code you copied is only meant to work within the Window_Status class, which isn't being used or modified by Sixth's script. Sixth's script instead uses some custom Window subclasses called Bio_Info and Bio_Bio that do not contain... well, a lot of the stuff that snippet you copied would need to work.
And from the error message, it seems you inserted it somewhere in Scene_SBio. If there was anywhere you would have needed to insert that code, it would have been in Bio_Bio. But it wouldn't have worked anyway - it's not Window_Status, it doesn't contain or call any Method called "draw_block3()" and if you tried to force it to run, you'd get a whole bunch of errors because stuff like "@actor" and "draw_equipments" don't exist within it anyway. It is a Window subclass so it does contain "line_height" and "draw_actor_param()" but doesn't have the stuff specific to Window_Status.
The way the default engine is structured that you have the Scene_ classes, which represent the different types of screens the player is shown. Scene_Title is the title screen, Scene_Battle is the battle screen, Scene_Map is the map, Scene_Menu the main menu, etc, etc. And within those screens, whenever some window that uses the windowskin is drawn, it's done by a separate piece of code labelled Window_something.
These Window_xxx classes are all subclasses built from the superclass named Window, which isn't actually visible in the script editor. It's one of the bits of code hidden in the RGSS3 dll and you need to check the help file to see what it does and what Methods are in it. The main thing that Window does is chop up the windowskin graphic file and arrange the pieces so it actually looks like a message window. The subclasses and sub-subclasses like Window_Selectable and Window_Command then add extra functionality like selectable buttons. Or are made to draw specific things. They can be as big as the whole screen like Window_Status, which draws all the parameter text. Or very small and only used for one thing one thing, like Window_Gold (which you know, just draws the party gold).
Sixth's script does the same thing, basically. He made a new Scene called Scene_SBio (and modified Scene_Menu's Window_MenuCommand to add a new button to open the Biography Menu) and a bunch of Window subclasses to contain all the text and faces and pictures and whatever.... except he didn't name them Window_xxx, he named them "Bio_Title" and "Bio_Info" and "Bio_Bio". You can still tell they're Window subclasses though, because they obviously show the windowskin and their superclass is Window_Base, a subclass of Window.
Now, where does all the text and data that they draw actually come from? Sometimes it's specified in the code of the window itself, other times it refers to other bits of code for it.
The stuff in the Biography Menu windows not specified in those windows themselves, they take it from the thing present at the top of the script - a Module called Sixth_Actor_Bio.
The contents of this Module are the "settings" that the user of the script is instructed to change in order to customize the script.
Unfortunately, that also means that this idea:
I'm guessing that I simply replace the text with the relevant command to display an actor's ATK/DEF, etc. stat but I can't find what that would be.
Will not work.
Irrespective of which slot they are placed in the script editor, Modules all get loaded before any of the classes (including Scene classes, so before you even see the title screen of the game). But the data you want to display - party member parameters - is all located within a class called Game_Party (or to be precise, an instance of the class that is located in a variable called "$game_party") and at the time when all the stuff in Sixth_Actor_Bio is being set up, Game_Party has not yet been created.
Your party literally does not exist when the settings for the script are being processed.
Thus trying to directly refer to "$game_party.members[0].atk" or anything similar in the script settings will just get you an error message.
If you want the windows in that menu to display that stuff, you will need to modify them to directly refer to the data in $game_party instead of reading the settings in Sixth_Actor_Bio. For reference, the individual party members within $game_party are Game_Actor objects.
Judging from your mockup, the window you want to modify is Bio_Bio. You can look at how Window_Status does it to get some idea of how parameter text is drawn and what to do.
Bio_Bio obviously does not do things quite the same way - for example, whereas Window_Status is given the actor that was selected by the player back in Scene_Menu and jumps directly to their page upon opening, the Biography Menu only ever starts on the party leader's page and its windows are hardcoded to initialize with the settings corresponding to the party leader. It will actually crash if you try to open it with zero party members/no leader (meanwhile Scene_Menu just greys out the Status option entirely if you have no members).
That said, the important thing to note is that, at the end of the day, all drawing of text in a window is ultimately done through a Method called "draw_text()" which all window classes inherit all the way down from the hidden Window superclass*. As long as you keep that in mind, you'll be able to force it to show something or other by playing around with it.
*EDIT: To be more precise, it's from the hidden Bitmap class, not Window. One of the things Window does is to create an instance of Bitmap called "contents", on which everything in the window (text, icons, gauges, etc) is drawn, usually by one of 3 Methods - either "draw_text()", "blt()" or "fill_rect()" or more rarely one of the variants of "blt()" and "fill_rect()" like "gradient_fill_rect()" in the case of some gauges.
