I see other have reported this error before in relation to the simulator.
I have just seen this error:
"The operation couldn't be completed (FBSOpenApplicationErrorDomain code =4)"
when trying to run a large playground file, after a one-line modification. I have deleted the new line, closed and opened the playground, closed and opened Xcode (7.2.1), but still see the same error.
Any thoughts?
I tried commenting out most of my code and just having a print("Hello World") line, but that produced the same error as well. So I opened a new playground and copied my code into it. It runs now. Not a very satisfactory solution but at least it allows me to continue working. I assume my playground file had become corrupted.