To select multiple projects to delete, you go to Your Stuff and then hove the mouse on a project. You will see a box on the left-side corner of the project thumbnails, as shown in the screenshot attached. You can tick the box and repeat the manipulation.
If you're having issues deleting, please refresh the page, delete the cookies from the browser, or change the browser.
If the issue persists, please share a screenshot so we can understand what you're experiencing.