If you mean "can't debug" is "cannot set break-point" then one possible reason is you haven't "activate" your extension yet. If this is the case, the extension source files has not been loaded yet. So that you obviously cannot set break-points.

To solve this, you can trigger ("activate") the extension by, for example run the extension command you defined. After this, this to set break-points again.

If you try to set break-points in "activate" code it-self, then properly you have to restart the debug session, though I haven't tested this yet.

Cheers.