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?
Mary couldn't (possibly) go to jail for the crime.