There is no source code available for the current location
Your project does not contain source code for the code you are trying to view. The usual cause is double-clicking a module that does not have source in the Call Stack Window or Threads Window. You can continue to debug, but cannot use the source window to set breakpoints and perform other actions at this location. If you need to set a breakpoint, use the Disassembly Window instead.
Another possible cause is trying to debug a dump file for managed code. Visual Studio supports debugging of managed minidumps in the Immediate window only, using the SOS tool. For information, see How to: Use SOS and SOS Debugging Extension (SOS.dll).