Out of Date After Effects Expression Undefined Value

Yeah, sure. Of course you will need to adjust the references to point to the correct controls for things to work.

Mylenium

The "Motion Controls" effect is probably a pseudo-effect created with a script or whatever which simply falls apart without the relevant XML bits being present in the respective file. Hard to say what it uses, but you may want to start by replacing the relevant bits with direct references to custom expression sliders. Also when adressing an index the value needs to be rounded to produce a proper integer. That would have been wrong even in the old code. That and of course your syntax for the conditionals is terrible and may similarly break whenever neither of the three defined conditions isn't met and there is no fallback. So more to the point, you code probably should look more like this:

mLay=thisComp.layer("Edit Settings");

mConA=mLay.effect("Slider A")("Slider");

mConB=mLay.effect("Slider B")("Slider");

x = thisComp.width/2 + mConA;

y = thisComp.height/2 + mConB;

defx = thisComp.width/2;

defy = thisComp.height/2;

m = Math.round(mLay.effect("Slider C")("Slider"));

if (m==1) {[x, defy]}

else if (m==2) {[defx, y]}

else if (m==3) {[x,y]}

else{[value[0],value[1]]}

Again, the code was terrible even for old AE and likely never worked reliably in the first place.

Mylenium

I think you might get that error if whatever thisComp.layer("Edit Settings").effect("Motion Controls")(1) is gets set to something greater than 3. The expression should probably check for that condition and fail more gracefully.