Issue with permalink

https://www.clarkillustrations.com/wp-content/uploads/2017/08/Capture.jpg

Hello, above is an image capture of the issue

I believe this was working before, and after the update it has an issue. The permalink needs to be “https://www.clarkillustrations.com/galleries/game-graphics/” but for some reason it won’t let me put the “/” in-between galleries and game-graphics. Also for some reason I can edit this one post’s permalink and not other post permalinks. I just want them to click my slideshow image and go straight to this page. I really don’t understand what the issue is here, please help?