Why does StyleCop output read "StyleCop cancelled"?

Take a look at the "Error List" tab. I'm pretty sure you'll find that there are 1000 warnings. If that number is reached, StyleCop stops. And I'm not aware that you can change that number.

Fix the style violations or disable the rules you don't want/need to get below 1000. Then StyleCop will run to the end.

StyleCop will be cancelled if the warnings are over 1000. Another problem is the ErrorList by default is showing the error for "Build Only"

Below is the step to make the warning appear: (*** Please note that The example below is using VS2017)

1) “StyleCop cancelled” with no reason states in the Output message:

2) Open the [Error List] tab and you will see nothing if the [Show issue generated] combo-box is "Build Only"

3) Change the [Show issue generated] combo-box to "Build + Intellisense":

4) If you still see nothing, maybe the filter is on, click the "Clear All Filters" button to clear all filters:

5) And you should be able to see all of the warnings now:

6) You may want to reduce the warnings by removing the unwanted rules, right-click on the project and choose “Properties.” In the screen that just opened, select “Code Analysis”:

6) By default, the “Microsoft Managed Recommended Rules” will be selected. Click on “Open” and you’ll see a long list of rules you can select or deselect: