[git] Why can I not switch branches?

Since the file is modified by both, Either you need to add it by

git add Whereami.xcodeproj/project.xcworkspace/xcuserdatauser.xcuserdatad/UserInterfaceState.xcuserstate

Or if you would like to ignore yoyr changes, then do

git reset HEAD Whereami.xcodeproj/project.xcworkspace/xcuserdatauser.xcuserdatad/UserInterfaceState.xcuserstate

After that just switch your branch.This should do the trick.