Modifiers and constraints use code within blender to perform their functionality. These don't export to other apps. You will need to either find a way to make it work without modifiers or make your xcode project move multiple bones at the same time.
The error you are getting is from having one chain of bones, the side bones are children of the bottom bones, so both of them move as you move their parents.
If you are exporting pre-set animations to use in your app, then you can export the animations with the modifier actions baked into the final animation. If you are dynamically moving the bones in your app, you will need to move multiple bones together.
You can have two chains of bones from the one root bone, like having a left and right arm of a character parented to the spine. In xcode you then need to move the lower side bone at the same time as the bottom bones as well as the left bottom bone to move sideways with the side bones.