That requires using the path and not just the name and file extension.
For commands like this it's much safer to
cd into the target directory and use the file name-extension instead of the full path. You're much less likely to make a mistake that can destroy something unintended.
convert command by design cannot edit a file in-place, so the syntax you were using could never work.
convert like this you have to provide the source and destination file names (or paths):
convert test.png -fill white -colorize 80% 'test copy.png'
On the other hand ImageMagick does provide a means to edit files in-place.
The command for this is
So – this macro follows the intent of the one you posted:
Whiten image.ccstone v1.00.kmmacros (6.3 KB)