My answer, I think, already replies to that.
If you’re purging FVM cache, your are deleting the files… so yeah, let it be and it will automatically cleanup unused files that are older than 45 days.
There should be no need to change any permissions, unless the files are never created on the first place.
When you first install FVM and refresh a page, there should be some css and js files created under wp-content/uploads/fvm
so if the fvm directory doesn’t exist at all, you can make sure that wp-content/uploads/
has 777 permissions (although usually 755 or 775 works fine).
If none of the files are being created, you can specify your cache path manually on the Pro tab, under Cache Location.
It shows you the “Default cache path” in use, so just look for what’s there and see if the files are being created. If not, add 777 and try again, using another browser (files may not be created if you’re logged in and selected the fix page editors option).
If you cannot make it work for some reason, then I recommend you to try another plugin, such as autoptimize.