I came across this page that teaches "could." https://www.englishpage.com/modals/could.html
The below example is from the page. I wonder whether "possibly" is a necessary element here to express future impossibility; because you see, if it is not there, it is a little unclear whether you are talking about the past ability or future possibility. However, all this is my guess. Can somebody shed some light on this?
Example 1
Mary couldn't (possibly) go to jail for the crime.