There is an issue loading Kendo UI for Angular packages in Stackblitz projects due to errors, related to the NGCC processing. Until those are resolved, the suggested approach is to create a new Angular 12 project locally, install all required packages, and copy-paste the app.component.ts and app.module.ts files (as well as any other files, required for the demo like products.ts).
After delving into the matter further, the problem seems to be caused by using RxJS 7, which is still not supported in Kendo UI for Angular packages, using a supported RxJS version resolves the issue:
Virtual Classroom, the free self-paced technical training that gets you up to speed with Telerik and Kendo UI products quickly just got a fresh new look + new and improved content including a brand new Blazor course! Check it out at https://learn.telerik.com/.