You can use the "$windowsfonts\" placeholder/variable as
destination path.
When you click on the "star" next to
destination, you see all DesktopCentral variables, under category
(drop down on top, right) "Disk & Folder" there is the
"Windows Fonts" variable.
The error you posted mentiones something about
"SrcDetails" = Source Details.
Something like
DC cannot access the source folder where the new fonts are saved
to (i.e. no permission on network share, maybe try http
upload)?
This is what it looks like in my config: