Is there an equivalent to Maya's Pin Bone?

aka I want a bone to stay in place, even while the other connected bones move around
Also, I can’t create other bones, I have to use the ones already on the model because reasons
(and yes I know I basically just asked this question but I can’t get IK to work for some reason)

There is a paid add-on called FreeIK, you can find it in the forum, that is for bone pinning.

I don’t know Maya.

You can use constraints to keep a bone from moving. For example, you can use a “Limit Location” constraint, set to world space, limiting the bone to a specific location in the world. (For rotation, it’s easier to just disable “inherit rotation” on the bone. Scale is probably not an issue, right?)

From what I remember of your old post, it seemed less like you wanted a bone to stay immobile, and more that you wanted only its tail to stay in the same position. Again, you can do this with a constraint. Probably a damped track, tracking an object placed at the location of the bone’s tail. Typically, this tracked object is another bone, but you don’t want to add bones, and you don’t have to: you can track an empty (or a mesh, or anything at all) if you’d prefer.

(Sometimes, you’ll want to combine a damped track with a locked track. Rather than explaining everything, I’m just going to encourage you to play with these constraints, because the way they work is pretty obvious once you start getting your hands dirty. You’ll have to make your own decision about how exactly you want your own personal skeleton to behave.)

It’s not clear why you can’t add bones. Many reasons would rule out other solutions people might offer. The solutions I’ve offered should work for baking visual keyframes for an exported animation, but Blender’s constraints will not be recognized by other engines. If that matters to you.