Creating a mklink to a folder on Windows 10

I have an application that writes files, let's say, on folder C:\Users\AppData\FOLDER

I want to create a link so the files end up on D:\FOLDER.

1. Do I use soft sym link or junction

2. Why do I have the error "cannot create a file when that file already exists?"

   mklink /D D:\FOLDER "C:\Users\AppData\Folder"

Both folders are on the same disk, they do exist, and they are different partitions with the same filesystem (NTFS)

Thanks in advance