Debugger watch expression vscode

I'm using the gdb debugger to debug c++ code. It works fine when I set breakpoints and view variables. A But I'm not able to Watch variables either in the watch column in the left or the debug console in the bottom.

As you can see in the images arr[q.front()] exists at the first break point. Still it shows not available and the debugger stops executing. Same when tried in the debug console. Is there only some kind of expressions which can be watched?