You need a valid file path. You are providing only a file name. You need something like “MyMac:Users:SomeUser:Desktop:MyQuarkDoc.qxd”. You may need to coerce it to an alias as well. I know QX 4.x you could “open newFileName as string”. If that doesn’t work then try “as alias”.
I’m curious as to why you are putting it in a property at the top of your script.