I’m trying to add skin to my character and wanted to apply subsurface scattering to give it more details. And yet, when I go to the BSDF node, it’s not there.
How do I get it?
hmmm it is right in your face 
(see your own screenshot again) and also you need to be in the rendered view with the Cycles renderer. Always check the manual for things like that https://docs.blender.org/manual/en/latest/render/shader_nodes/shader/principled.html
Oh! Are you saying that the options Christensen-Burley, Random Walk, and Random Walk (Skin) are the Subsurface scattering?
No, if you read the manual it says that if you set the SSS “Weight” parameter slider to 1.0 (btw it already is in your image) you get the subsurface scattering effect, but you are not in rendered view mode so you can’t see any effect yet on your screen blender…
and if is too strong the SSS effect tweak the scale parameter to decrease it.
Also watch these tutorials for more advanced tweaking and/or understanding of this:
Thank you for the tutorials! I’ll be sure to check them out.
I switched to Render view and I can see the effects now. (My laptop is being slow though, but that’s neither here or there. 
)