You have to be signed in to your Google account. Some users with Google Accounts hosted by schools or companies don’t have full permissions to use files from other domains in order to protect the students/employees. If that is your case please try one of the following:
- Use a personal Google account
- Ask your company/school to whitelist files from url https://www.slidescarnival.com or domain SlidesCarnival.com
- Remember that you can always download the PowerPoint file and upload it to your Google Drive to edit with Google Slides, it’s a workaround but it should work as desired.