Can somebody specify where this folder should exist?
I created it in the directory containing the vendor directory, in the files directory, outside of the files and in the site root directory, but I keep getting this error message.
Writable (public download method)
The directory ../private is not writable. An automated attempt to create this directory failed, possibly due to a permissions problem. To proceed with the installation, either create the directory and modify its permissions manually or ensure that the installer has the permissions to create it automatically. For more information, see INSTALL.txt or the online handbook.