Open your Finder. Grab the Application folder from the side bar with your mouse or trackpad and drag it and drop it on the RIGHT side of the divider line of the Dock.
What divider line?
Look closely at your Dock. On the right side of the Dock you'll see a thin black line on it. Drop the Applications folder to the right of that line. Let us know if that works for you.
Right click on the Application folder on the Dock that you replaced and select: Name, Folder, List. That should get your folder back to normal. Also I don't know why you're not seeing the line unless you've got your Dock on the sides instead of on the bottom of your display.
TMy mini is a few years (3?) old. Maybe there was no black line then?