Hello Eleonora,
Can you make sure you are only logged into one Google account ( multiple accounts open can cause issues) and that this file is on your drive ( not a team drive).
You can also try and open autocrat in an incognito window.
If this issue still persists, can you hold down Control+Shift+J" on a PC or "Command+Option+I" on a Mac and when the side window opens, take a screenshot of the error console ( make sure the side window is on the 'console' tab)
Also, go the Autocrat menu in the add-ons drop down and select 'User key', we can check if any additional errors are being logged on the backend.
Thanks!