Unable to create folder in root of 'Macintosh HD'?

In Catalina the primary volume is read-only. This means it is not immediately possible to create new folders here.

In order to get around that, the system provides what is known as synthetic firm links. This allows you to create what appears to be folders at the root of the file system.

You need to create the file /etc/synthetic.conf, which should be owned by root and group wheel with permissions 0644.

The contents should look like this:

newfolder     Users/foo/bar

"newfolder" is the name of the virtual folder that will be created in the root of the file system. "Users/foo/bar" is the actual location of the folder. You need to ensure that this folder actually exists. It doesn't need to be inside Users, but can be anywhere in your system.

NOTE: It is important to ensure that the space between the two folder names is a TAB character, and not just a number of space.

After creating the file above with the specified contents, you need to reboot the system. After rebooting, you'll see the /newfolder folder.