error: failed to attach to process ID 0

Go to IOS Simulator tool bar and click Reset Content and Settings...

I've met the same problem

1.you can delete the App directy under

~/Library/Application Support/iPhone Simulator/6.0/Applications

and

~/Library/Developer/Xcode/DerivedData

2.then you can start Xcode ; in the menubar you can find Project -> Clean

EDIT: before copy/pasting 1 above change the Simulator Version to what you are using!

Click on Simulator and reset the simulator by going to the “iOS Simulator” main menu and choose “Reset Content and Settings…

Back in XCode, choose the menu "Product" > "Edit Scheme". On the left menu panel, click on "Run _Your_Project_Name_.app" > "Run _Your_Project_Name.app". The info display on the right should have the "Debug Process As" radio button picked as "Me (your-user-name)". If not, make sure you do not login as root. On Stackoverflow there are many reports of running as root will have problems

Next, still in Xcode, go to the menu "Window" > "Organizer". On the top menu, click on "Projects". Go ahead and delete "Derived Data".

Finally, go to "~/Library/Application Support/iPhone Simulator/6.0/Applications" and delete everything that’s in there