Can't add some shortcuts folders to the dock
Operating systems can sometimes do strange things. One of the things I've seen with macOS when it comes to the Dock is that it can sort of glitch when you've been adding lots of items. This seems to happen moreso with folders (especially if you've added one, removed it, and then tried to add it back again).

What always resolves this glitch when I see it, is a simple reboot of the Mac. After rebooting, it accepts the folder. So I'd try restarting your Mac and seeing if that's what the problem is in this case.