There are no slashes in the dates, so you should not use them in the definition of the format within as.Date(). Use spaces and a comma as in your data. For example
as.Date("September 25, 1789", format = "%B %d, %Y")
@sarahharvesyou posted here before with screenshots instead of properly accessible code or ideally a reprex. As a newcomer to the forum you were helped out anyway, but you did not seem to take notice of the requests by those who were helping you.
This isn't your first time here so you should be aware of the importance of providing better material to people to work with. You are, after all, asking people to give up their free time to help you, so it makes sense that you should help them to help you.