This usually happens when the presentation is still a .pptx file inside Google Slides instead of a native Google Slides file. Aligned works best with a native Google Slides link.
How to fix it
Open the presentation in Google Slides.
Make sure the file is saved as a native Google Slides presentation, not a .pptx file.
Click Share and set access to Anyone with the link.
Copy the updated Google Slides link.
Go back to your Room and replace the old link.
Click Preview and check that the presentation loads correctly.
Still not working?
Make sure you copied the link from the native Google Slides file, not the original .pptx file.
Make sure the file is shared as Anyone with the link.
Replace the link in the Room and test it again in Preview.